## Rethinking linear algebra: visualizing linear transformations and eigenvectors

In terms of calculation processes of Principal Component Analysis (PCA) or Linear Discriminant Analysis (LDA), which are the dimension reduction techniques I am going to explain in the following articles, diagonalization is what they are all about. Throughout this article, I would like you to have richer insight into diagonalization in order to prepare for understanding those basic dimension reduction techniques.

When our professor started a lecture on the last chapter of our textbook on linear algebra, he said “It is no exaggeration to say that everything we have studied is for this ‘diagonalization.'” Until then we had to write tons of numerical matrices and vectors all over our notebooks, calculating those products, adding their rows or columns to other rows or columns, sometimes transposing the matrices, calculating their determinants.

It was like the scene in “The Karate Kid,” where the protagonist finally understood the profound meaning behind the prolonged and boring “wax on, wax off” training given by Miyagi (or “jacket on, jacket off” training given by Jackie Chan). We had finally understood why we had been doing those seemingly endless calculations.

But usually you can do those calculations easily with functions in the Numpy library. Unlike Japanese college freshmen, I bet you are too busy to reopen textbooks on linear algebra to refresh your mathematics. Thus I am going to provide less mathematical and more intuitive explanation of diagonalization in this article.

*This is the second article of the article series ” Illustrative introductions on dimension reduction .”

### 1, The mainstream ways of explaining diagonalization.

*The statements below are very rough for mathematical topics, but I am going to give priority to offering more visual understanding on linear algebra in this article. For further understanding, please refer to textbooks on linear algebra. If you would like to have minimum understandings on linear algebra needed for machine learning, I recommend the Appendix C of Pattern Recognition and Machine Learning by C. M. Bishop.

In most textbooks on linear algebra, the explanations on dioagonalization is like this (if you are not sure what diagonalization is or if you are allergic to mathematics, you do not have to read this seriously):

Let be a vector space and let  be a mapping of into itself,  defined as , where is a matrix and is dimensional vector. An element is called an eigen vector if there exists a number such that and . In this case is uniquely determined and is called an eigen value of belonging to the eigen vector .

Any matrix has eigen values , belonging to . If is basis of the vector space , then is diagonalizable.

When is diagonalizable, with matrices , whose column vectors are eigen vectors , the following equation holds: , where .

And when is diagonalizable, you can diagonalize as below.

Most textbooks keep explaining these type of stuff, but I have to say they lack efforts to make it understandable to readers with low mathematical literacy like me. Especially if you have to apply the idea to data science field, I believe you need more visual understanding of diagonalization. Therefore instead of just explaining the definitions and theorems, I would like to take a different approach. But in order to understand them in more intuitive ways, we first have to rethink waht linear transformation means in more visible ways.

### 2, Linear transformations

Even though I did my best to make this article understandable to people with little prerequisite knowledge, you at least have to understand linear transformation of numerical vectors and with matrices. Linear transformation is nothing difficult, and in this article I am going to use only 2 or 3 dimensional numerical vectors or square matrices. You can calculate linear transformation of by as equations in the figure. In other words, is a vector transformed by .

*I am not going to use the term “linear transformation” in a precise way in the context of linear algebra. In this article or in the context of data science or machine learning, “linear transformation” for the most part means products of matrices or vectors.

*Forward/back propagation of deep learning is mainly composed of this linear transformation. You keep linearly transforming input vectors, frequently transforming them with activation functions, which are for the most part not linear transformation.

As you can see in the equations above, linear transformation with transforms a vector to another vector. Assume that you have an original vector in grey and that the vector in pink is the transformed by is. If you subtract from , you can get a displacement vector, which I displayed in purple. A displacement vector means the transition from a vector to another vector.

Let’s calculate the displacement vector with more vectors . Assume that , and I prepared several grid vectors in grey as you can see in the figure below. If you transform those grey grid points with , they are mapped into the vectors in pink. With those vectors in grey or pink, you can calculate the their displacement vectors , which are in purple.

The displacement vectors in the figure above have some tendencies. In order to see that more clearly, let’s calculate displacement vectors with several matrices and more grid points. Assume that you have three square matrices , and I plotted displace vectors made by the matrices respectively in the figure below.

I think you noticed some characteristics of the displacement vectors made by those linear transformations: the vectors are swirling and many of them seem to be oriented in certain directions. To be exact, some displacement vectors extend in the same directions as some of original vectors in grey. That means  linear transformation by did not change the direction of the original vector , and the unchanged vectors are called eigen vectors. Real eigen vectors of each A are displayed as arrows in yellow in the figure above. But when it comes to , the matrix does not have any real eigan values.

In linear algebra, depending on the type matrices , you have to consider various cases such as whether the matrices have real or imaginary eigen values, whether the matrices are diagonalizable, whether the eigen vectors are orthogonal, or whether they are unit vectors. But those topics are out of the scope of this article series, so please refer to textbooks on linear algebra if you are interested.

Luckily, however, in terms of PCA or LDA, you only have to consider a type of matrices named positive semidefinite matrices, which is classified to, and I am going to explain positive semidefinite matrices in the fourth section.

### 3, Eigen vectors as coordinate system

Source: Ian Stewart, “Professor Stewart’s Cabinet of Mathematical Curiosities,” (2008), Basic Books

Let me take Fibonacci numbers as an example to briefly see why diagonalization is useful. Fibonacci is sequence is quite simple and it is often explained using an example of pairs of rabbits increasing generation by generation. Let be the number of pairs of grown up rabbits in the generation. One pair of grown up rabbits produce one pair of young rabbit The concrete values of are , , , , , , , . Assume that and that , then you can calculate the number of the pairs of grown up rabbits in the next generation with the following recurrence relation. .Let be , then the recurrence relation can be written as , and the transition of are like purple arrows in the figure below. It seems that the changes of the purple arrows are irregular if you look at the plots in normal coordinate.

Assume that are eigen values of , and are eigen vectors belonging to them respectively. Also let scalars such that . According to the definition of eigen values and eigen vectors belonging to them, the following two equations hold: . If you calculate is, using eigen vectors of , . In the same way, , and . These equations show that in coordinate system made by eigen vectors of , linear transformation by is easily done by just multiplying eigen values with each eigen vector. Compared to the graph of Fibonacci numbers above, in the figure below you can see that in coordinate system made by eigen vectors the plots changes more systematically generation by generation.

In coordinate system made by eigen vectors of square matrices, the linear transformations by the matrices can be much more straightforward, and this is one powerful strength of eigen vectors.

*I do not major in mathematics, so I am not 100% sure, but vectors in linear algebra have more abstract meanings. Various things in mathematics can be vectors, even though in machine learning or data science we  mainly use numerical vectors with more concrete elements. We can also say that matrices are a kind of maps. That is just like, at least in my impression, even though a real town is composed of various components such as houses, smooth or bumpy roads, you can simplify its structure with simple orthogonal lines, like the map of Manhattan. But if you know what the town actually looks like, you do not have to follow the zigzag path on the map.

### 4, Eigen vectors of positive semidefinite matrices

In the second section of this article I told you that, even though you have to consider various elements when you discuss general diagonalization, in terms of PCA and LDA we mainly use only a type of matrices named positive semidefinite matrices. Let be a square matrix. If for all values of the vector , the is said to be a positive semidefinite matrix. And also it is known that being a semidefinite matrix is equivalent to for all the eigen values .

*I think most people first learn a type of matrices called positive definite matrices. Let be a square matrix. If for all values of the vector , the is said to be a positive definite matrix. You have to keep it in mind that even if all the elements of are positive, is not necessarly positive definite/semidefinite.

Just as we did in the second section of this article, let’s visualize displacement vectors made by linear transformation with a square positive semidefinite matrix .

*In fact , whose linear transformation I visualized the second section, is also positive semidefinite.

Let’s visualize linear transformations by a positive definite matrix . I visualized the displacement vectors made by the just as the same way as in the second section of this article. The result is as below, and you can see that, as well as the displacement vectors made by , the three dimensional displacement vectors below are swirling and extending in three directions, in the directions of the three orthogonal eigen vectors , and .

*It might seem like a weird choice of a matrix, but you are going to see why I chose it in the next article.

You might have already noticed and are both symmetric matrices and that their elements are all real values, and that their diagonal elements are all positive values. Super importantly, when all the elements of a symmetric matrix are real values and its eigen values are , there exist orthonormal matrices such that , where .

*The title of this section might be misleading, but please keep it in mind that positive definite/semidefinite matrices are not necessarily real symmetric matrices. And real symmetric vectors are not necessarily positive definite/semidefinite matrices.

### 5, Orthonormal matrices and rotation of vectors

In this section I am gong to explain orthonormal matrices, as known as rotation matrices. If a matrix is an orthonormal matrix, column vectors of are orthonormal, which means , where . In other words column vectors form an orthonormal coordinate system.

Orthonormal matrices have several important properties, and one of the most important properties is . Combining this fact with what I have told you so far, you we can reach one conclusion: you can orthogonalize a real symmetric matrix as . This is known as spectral decomposition or singular value decomposition.

Another important property of is that is also orthonormal. In other words, assume is orthonormal and that , also forms a orthonormal coordinate system.

…It seems things are getting too mathematical and abstract (for me), thus for now I am going to wrap up what I have explained in this article .

We have seen

• Numerical matrices linearly transform vectors.
• Certain linear transformations do not change the direction of vectors in certain directions, which are called eigen vectors.
• Making use of eigen vectors, you can form new coordinate system which can describe the linear transformations in a more straightforward way.
• You can diagonalize a real symmetric matrix with an orthonormal matrix .

Of our current interest is what kind of linear transformation the real symmetric positive definite matrix enables. I am going to explain why the purple vectors in the figure above is swirling in the upcoming articles. Before that, however, we are going to  see one application of what we have seen in this article, on dimension reduction. To be concrete the next article is going to be about principal component analysis (PCA), which is very important in many fields.

*In short, the orthonormal matrix , which I mentioned above enables rotation of matrix, and the diagonal matrix expands or contracts vectors along each axis. I am going to explain that more precisely in the upcoming articles.

* I make study materials on machine learning, sponsored by DATANOMIQ. I do my best to make my content as straightforward but as precise as possible. I include all of my reference sources. If you notice any mistakes in my materials, including grammatical errors, please let me know (email: yasuto.tamura@datanomiq.de). And if you have any advice for making my materials more understandable to learners, I would appreciate hearing it.

*I attatched the codes I used to make the figures in this article. You can just copy, paste, and run, sometimes installing necessary libraries.

## Zertifikatsstudium „Data Science and Big Data“

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## Connections Between Data Science & Finance

Image Source: pixabay.com

The world of finance is changing at an unprecedented rate. Data science has completely altered the face of traditional finance management. Though data has long been a critical component to finances, the introduction of big data and artificial intelligence have created new tools that are strengthening the predictive ability of many financial institutions.

These changes have led to a rapid increase in the need for financial professionals with data science skills. Nearly every sector in finances is converting to greater use of data science and management from the stock market and retirement accounts to credit score calculation. A greater understanding of the interplay between data and finance is a key skill gap.

Likewise, they have opened many doors for those that are interested in analyzing their personal finances. More and more people are taking their finances into their own hands and using the data tools available to make the best decisions for them. In today’s world, the sky’s the limit for financial analysis and management!

## The Rise of the Financial Analyst

Financial analysts are the professionals who are responsible for the general management of money and investments both in an industrial and personal finance realm. Typically a financial analyst will spend time reviewing and understanding the overall stock portfolio and financial standing of a client including:

• Stocks
• Bonds
• Retirement accounts
• Financial history
• Current financial statements and reports
• Overarching business and industry trends

From there, the analyst will provide a recommendation with data-backed findings to the client on how they should manage their finances going into the future.

As you can imagine, with all of this data to analyze, the need for financial analysts to have a background or understanding of data science has never been higher! Finance jobs requiring skills such as artificial intelligence and big data increased by over 60% in the last year. Though these new jobs are typically rooted in computer science and data analytics, most professionals still need a background in financial management as well.

The unique skills required for a position like this means there is a huge (and growing) skills gap in the financial sector. Those professionals that are qualified and able to rise to fill the need are seeing substantial pay increases and hundreds of job opportunities across the nation and the globe.

## A Credit Score Example

But where does all of this data science and professional financial account management come back to impact the everyday person making financial decisions? Surprisingly, pretty much in every facet of their lives. From things like retirement accounts to faster response times in financial analysis to credit scores — data science in the financial industry is like a cloaked hand pulling the strings in the background.

Take, for example, your credit score. It is one of the single most important numbers in your life, for better or worse. A high credit score can open all sorts of financial doors and get you better interest rates on the things you need loans for. A bad score can limit the amount lenders willing to qualify you for a loan and increase the interest rate substantially, meaning you will end up paying far more money in the end.

Your credit score is calculated by several things — though we understand the basic outline of what goes into the formula, the finer points are somewhat of a mystery. We know the big factors are:

• Personal financial history
• Debit-credit ratio
• Length of credit history
• Number of new credit hits or applications

All of this data and number crunching can have a real impact on your life, just one example of how data in the financial world is relevant.

## Using Data Science in Personal Finance

Given all this information, you might be thinking to yourself that what you really need is a certificate in data science. Certainly, that will open a number of career doors for you in a multitude of realms, not just the finance industry. Data science is quickly becoming a cornerstone of how most major industries do business.

However, that isn’t necessarily required to get ahead on managing your personal finances. Just a little information about programs such as Excel can get you a long way. Some may even argue that Excel is the original online data management tool as it can be used to do things like:

• Create schedules
• Manage budgets
• Visualize data in charts and graphs
• Track revenues and expenses
• Conditionally format information
• Manage inventory
• Identify trends in large data sets

There are even several tools and guides out there that will help you to get started!

***

Data analysis and management is here to stay, especially when it comes to the financial industry. The tools are likely to continue to become more important and skills in their use will increase in value. Though there are a lot of professional skills using big data to manage finances, there are still a lot of tools out there that are making it easier than ever to glean insights into your personal finances and make informed financial decisions.

## K Nearest Neighbour For Supervised Learning

K-Nearest Neighbour (KNN) Algorithms is an easy-to-implement & advanced level supervised machine learning algorithm used for both – classification as well as regression problems. However, you can see a wide of its applications in classification problems across various industries.

If you’ve been shopping a lot in e-commerce sites like Amazon, Flipkart, Myntra, or love watching web series over Netflix and Amazon Prime, one common thing you’ve always noticed, and that is recommendations.

Are you wondering how they recommend you following your choice? They use KNN Supervised Learning to find out what you may need the next when you’re buying and recommend you with a few more products.

Imagine you’re looking for an iPhone to purchase. When you scroll down a little, you see some iPhone cases, tempered glasses – saying, “People who purchased an iPhone have also purchased these items. The same applies to Netflix and Amazon Prime. When you finished a show or a series, they give you recommendations of the same genre. And do it all using KNN supervised learning and classify the items for the best user experience.

• Quickest Calculation Time
• Simple Algorithms
• High Accuracy
• Versatile – best use for Regression and Classification.
• Doesn’t make any assumptions about data.

## Where KNN Are Mostly Used

• Simple Recommendation Models
• Image Recognition Technology
• Decision-Making Models
• Calculating Credit Rating

## Choosing The Right Value For K

To choose the right value of K, you have to run KNN algorithms several times with different values of K and select the value of K, which reduces the number of errors you’ve come across and come out as the most stable value for K.

## Your Step-By-Step Guide For Choosing The Value Of K

• As you decrease the value of K to 1 (K = 1), you’ll reach a query point, where you get to see many elements from class A (-) and class B (+) where (-) is the only nearest neighbor. Reasonably, you would think about the query point to be most likely the red one. As K =1, which has a blue color, KNN incorrectly predicts the wrong color blue.
• As you increase the value of K to 2 (K=2), you get to see two elements, (-) and (+) are the only nearest neighbor. As you have two values, which are of Class A and Class B, KNN incorrectly predicts the wrong values (Blue and Red).
• As you increase the value of K to 3 (K=3), you get to see three elements (-) and (+), (+) are the only nearest neighbor. And this time, you got three values, one from blue and two from red. As your assumption is red, KNN correctly predicts the right value (Blue and Red, Red). Your answer is more stable this time compared to previous ones.

## Conclusion

KNN works by finding the nearest distance between a query and all the elements in the database. By choosing the value for K, we get the closest to the query. And then, KNN algorithms look for the most frequent labels in classification and averages of labels in regression.

## Spiky cubes, Pac-Man walking, empty M&M’s chocolate: curse of dimensionality

This is the first article of the article series Illustrative introductions on dimension reduction.

“Curse of dimensionality” means the difficulties of machine learning which arise when the dimension of data is higher. In short if the data have too many features like “weight,” “height,” “width,” “strength,” “temperature”…., that can undermine the performances of machine learning. The fact might be contrary to your image which you get from the terms “big” data or “deep” learning. You might assume that the more hints you have, the better the performances of machine learning are. There are some reasons for curse of dimensionality, and in this article I am going to introduce two major reasons below.

1. High dimensional data usually have rich expressiveness, but usually training data are too poor for that.
2. The behaviors of data points in high dimensional space are totally different from our common sense.

Through these topics, you will see that you always have to think about which features to use considering the number of data points.

*From now on I am going to talk about only Euclidean distance. If you are not sure what Euclidean distance means, please just keep it in mind that it is the type of distance most people wold have learnt in normal compulsory education.

*This is the first article of the article series ” Illustrative introductions on dimension reduction .”

### 1. Number of samples and degree of dimension

The most straightforward demerit of adding many features, or increasing dimensions of data, is the growth of computational costs. More importantly, however, you always have to think about the degree of dimensions in relation of the number of data points you have. Let me take a simple example in a book “Pattern Recognition and Machine Learning” by C. M. Bishop (PRML). This is an example of measurements of a pipeline. The figure below shows a comparison plot of 3 classes (red, green and blue), with parameter plotted against parameter out of 12 parameters.

* The meaning of data is not important in this article. If you are interested please refer to the appendix in PRML.

Assume that we are interested in classifying the cross in black into one of the three classes. One of the most naive ideas of this classification is dividing the graph into grids and labeling each grid depending on the number of samples in the classes (which are colored at the right side of the figure). And you can classify the test sample, the cross in black, into the class of the grid where the test sample is in. Thereby the cross is classified to the class in red.

Source: C.M. Bishop, “Pattern Recognition and Machine Learning,” (2006), Springer, pp. 34-35

As I mentioned in the figure above, we used only two features out of 12 features in total. When the total number of data points is fixed and you add remaining ten axes/features one after another, what would happen? Let’s see what “adding axes/features” means. If you are talking about 1, 2, or 3 dimensional grids, you can visualize them. And as you can see from the figure below, if you make each grids respectively in 1, 2, 3 dimensional spaces, the number of the small regions in the grids are respectively 10, 100, 1000. Even though you cannot visualize it anymore, you can make grids for more than 3 dimensional data. If you continue increasing the degree of dimension, the number of grids increases exponentially, and that can soon surpass the number of training data points. That means there would be a lot of empty spaces in such high dimensional grids. And the classifying method above: coloring each grid and classifying unknown samples depending on the colors of the grids, does not work out anymore because there would be a lot of empty grids.

* If you are still puzzled by the idea of “more than 3 dimensional grids,” you should not think too much about that now. It is enough if you can get some understandings on high dimensional data after reading the whole article of this.

Source: Goodfellow and Yoshua Bengio and Aaron Courville, Deep Learning, (2016), MIT Press, p. 153

I said the method above is the most naive way, but other classical classification methods , for example k-nearest neighbors algorithm, are more or less base on a similar idea. Many of classical machine learning algorithms are based on the idea of smoothness prior, or local constancy prior. In short in classical ways, you  do not expect data to change so much in a small region, so you can expect unknown samples to be similar to data in vicinity. But that soon turns out to be problematic when the dimension of data is bigger because training data would be sparse because the area of multidimensional space grows exponentially as I mentioned above. And sometimes you would not be able to find training data around test data. Plus, in high dimensional data, you cannot treat distance in the same as you do in lower dimensional space. The ideas of “close,” “nearby,” or “vicinity” get more obscure in high dimensional data. That point is related to the next topic: the intuition have cultivated in normal life is not applicable to higher dimensional data.

### 2. Bizarre characteristics of high dimensional data

We form our sense of recognition in 3-dimensional ways in our normal life. Even though we can visualize only 1, 2, or 3 dimensional data, we can actually generalize the ideas in 1, 2, or 3 dimensional ideas to higher dimensions. For example 4 dimensional cubes, 100 dimensional spheres, or orthogonality in 255 dimensional space. Again, you cannot exactly visualize those ideas, and for many people, such high dimensional phenomenon are just imaginary matters on blackboards. Those high dimensional ideas are designed to retain some conditions just as well as 1, 2, or 3 dimensional space. Let’s take an example of spheres in several dimensional spaces. General spheres in any D-dimensional space can be defined as a set of any , such that , where is the center point and is length of radius. When is 2-dimensional, the spheres are called “circles.” When is 3-dimensional, the spheres are called “spheres” in our normal life, unless it is used in a conversation in a college cafeteria, by some students in mathematics department. And when is D-dimensional, they are called D-ball, and again, this is just a imaginary phenomenon on blackboard.

* Vectors and points are almost the same because all the vectors are denoted as “arrows” from the an origin point to sample data points.  The only difference is that when you use vectors, you have to consider their directions.

* “D-ball” is usually called “n-ball,” and in such context it is a sphere in a n-dimensional space. But please let me use the term “D-ball” in this article.

Not only spheres, but only many other ideas have been generalized to D-dimensional space, and many of them are indispensable also for data science. But there is one severe problem: the behaviors of data in high dimensional field is quite different from those in two or three dimensional space. To be concrete, in high dimensional field, cubes are spiky, you have to move like Pac-Man, and M & M’s Chocolate looks empty inside but tastes normal.

2.1: spiky cubes
Let’s take a look at an elementary-school-level example of geometry first. Assume that you have several unit squares or unit cubes like below. In each of them a circle or sphere with diameter 1 is inscribed. The length of a diagonal line in each square is , and that in each cube is .

If you stack the squares or cubes as below, what are the length of diameters of the blue circle or sphere, circumscribing all the 4 orange circles or the 8 orange spheres?

The answers are, the diameter of the blue circle is , and the diameter of the blue sphere is .

Next let’s think about the same situation in higher dimensional space. Assume that there are some unit D-dimensional hypercubes stacked, in each of which a D-ball with diameter 1 is inscribed, touching all the surfaces inside. Then what is the length of the diameter of  a D-ball circumscribing all the unit D-ball in the hypercubes ? Given the results above, it ca be predicted that its diameter is . If that is true, there is one strange point: can soon surpass 2: that means in the chart above the blue sphere will stick out of the stacked cubes. That sounds like a paradox, but with one hypothesis, the phenomenon makes sense: cubes become more spiky as the degree of dimension grows. This hypothesis is a natural deduction because diagonal lines of hyper cubes get longer, and the the center of each surface of hypercubes still touches the unit D-ball with diameter 1, inscribing inscribing inside each unit hypercube.

If you stack 4 hypercubes, the blue sphere circumscribing them will not stick out of the stacked hypercubes anymore like the figure below.

*Of course you cannot visualize what is going on in D-dimensional space, so the figure below is just a pseudo simulation of D-dimensional space in our 3-dimensional sense. I guess you have to stack more than four hyper cubes in higher dimensional data, but you cannot easily imagine what will go on in such space anymore.

*You can confirm the fact that hypercube gets more spiky as the degree of dimension growth, by comparing the volume of the hypercube and the volume of the D-ball inscribed inside the hypercube. Thereby you can prove that the volume of hypercube concentrates on the corners of the hypercube. Plus, as I mentioned the longest diagonal distance of hypercube gets longer as dimension degree increases. That is why hypercube is said to be spiky. For mathematical proof, please check the Exercise 1.19 of PRML.

#### 2.2: Pac-Man walking

Next intriguing phenomenon in high dimensional field is that most of pairs of vectors in high dimensional space are orthogonal. In other words, if you select two random vectors in high dimensional space, the angle between them are mostly close to . Let’s see the general meaning of angle between two vectors in any dimensional spaces. Assume that the angle between two vectors , and is , then is calculated as . In 1, 2, or 3 dimensional space, you can actually see the angle, but again you can define higher dimensional angle, which you cannot visualize anymore. And angles are sometimes used as similarity of two vectors.

* is the inner product of , and .

Assume that you generate a pair of two points inside a D-dimensional unit sphere and make two vectors , and by connecting the origin point and those two points respectively. When D is 2, I mean spheres are circles in this case, any are equally generated as in the chart below. The fact might be the same as your intuition.   How about in 3-dimensional space? In fact the distribution of is not uniform. is the most likely to be generated. As I explain in the figure below, if you compare the area of cross section of a hemisphere and the area of a cone whose vertex is the center point of the sphere, you can see why.

I generated 10000 random pairs of points in side a D-dimensional unit sphere, and calculated the angle between them. In other words I just randomly generated two D-dimensional vectors and , whose elements are randomly generated values between -1 and 1, and calculated the angle between them, repeating this process 10000 times. The chart below are the histograms of angle between pairs of generated vectors in respectively 2, 3, 50, and 100 dimensional space.

As I explained above, in 2-dimensional space, the distribution of is almost uniform. However the distribution concentrates a little around in 3-dimensional space. You can see that the bigger the degree of dimension is, the more the angles of generated vectors concentrate around . That means most pairs of vectors in high dimensional space are close to orthogonal. Movements are also sequence of vectors, so when most pairs of movement vectors are orthogonal, that means you can only move like Pac-Man in such space.

Source: https://edition.cnn.com/style/article/pac-man-40-anniversary-history/index.html

* Of course I am talking about arcade Mac-Man game. Not Pac-Man in Super Smash Bros.  Retro RPG video games might have more similar playability, but in high dimensional space it is also difficult to turn back. At any rate, I think you have understood it is even difficult to move smoothly in high dimensional space, just like the first notorious Resident Evil on the first PS console also had terrible playability .

#### 2.3: empty M & M’s chocolate

Let’s think about the proportion of the volume of the outermost surface of general spheres with radius First, in 2 two dimensional space, spheres are circles. The area of the brown part of the circle below is . In order calculate the are of thick surface of the circle, you have only to subtract the area of . When , the area of outer most surface is , and its proportion to the area of the whole circle is .

In case of 3-dimensional space, the value of a sphere with radius is , so the proportion of the surface is calculated in the same way: . Compared to the case in 2 dimensional space, the proportion is a little bigger.

How about in D-dimensional space? We have seen that even in  D-dimensional space the surface of a sphere, I mean D-ball, can be defined as a set of any points whose distance from the center point is all . And it is known that the volume of D-ball is defined as below.

is called gamma function, but in this article it is not so important. The most important point now is, if you discuss any D-ball, their volume only depends on their radius . That meas the proportion of outer surface of D-ball is calculated as . When is 0.01, the proportion of the 1% surface of D-ball changes like in the chart below.

* And of course when is 2,  , and when is 3 ,

You can see that when D is over 400, around 90% of volume is concentrated in the very thin 1% surface of D-ball. That is why, in high dimensional space, M & M’s chocolate look empty but tastes normal: all the chocolate are concentrated beneath the sugar coating.

More interestingly, even if you choose any points as a central point of a sphere with radius , the other points are squashed to the surface of the sphere, even if all the data points are uniformly distributed. This situation is problematic for classical machine learning algorithms, which are often based on the Euclidean distances between pairs of two sample data points: if you go from the central point to another sample point, the possibility of finding the point within radius of the center is almost zero. But if you reach the outermost part of the surface of the sphere, most data points are there. However, for one of the data points in the surface, any other data points are distant in the same way.

Inside M & M’s chocolate is a mysterious world.

Source: https://hipwallpaper.com/mms-wallpapers/

You have seen that using high dimensional data can be problematic in many ways. Data science and machine learning are largely based on one idea: you can find a lower dimensional meaningful and easier structure in data. In the next articles I am going to introduce some famous dimension reduction algorithms. And hopefully I would like to give some deeper insights in to these algorithms, in straightforward ways.

* I could not explain the relationships of variance and bias of data. This is also a very important factor when you think about dimensionality of data. I hope I can write about this topic someday. You can also look it up if you are interested.

### [References]

[1]C. M. Bishop, “Pattern Recognition and Machine Learning,” (2006), Springer, pp. 33-37

[2]Goodfellow and Yoshua Bengio and Aaron Courville, Deep Learning, (2016), MIT Press, p. 153

[3] Shiga Kouji, “30 Lesson to Topology,” (1988)

[4]”Volume of an n-ball,” Wikipedia
https://en.wikipedia.org/wiki/Volume_of_an_n-ball

* I make study materials on machine learning, sponsored by DATANOMIQ. I do my best to make my content as straightforward but as precise as possible. I include all of my reference sources. If you notice any mistakes in my materials, including grammatical errors, please let me know (email: yasuto.tamura@datanomiq.de). And if you have any advice for making my materials more understandable to learners, I would appreciate hearing it.

## Illustrative introductions on dimension reduction

“What is your image on dimensions?”

….That might be a cheesy question to ask to reader of Data Science Blog, but most people, with no scientific background, would answer “One dimension is a line, and two dimension is a plain, and we live in three-dimensional world.” After that if you ask “How about the fourth dimension?” many people would answer “Time?”

You can find books or writings about dimensions in various field. And you can use the word “dimension” in normal conversations, in many contexts.

*In Japanese, if you say “He likes two dimension.” that means he prefers anime characters to real women, as is often the case with Japanese computer science students.

The meanings of “dimensions” depend on the context, but in data science dimension is usually the number of rows of your Excel data.

When you study data science or machine learning, usually you should start with understanding the algorithms with 2 or 3 dimensional data, and you can apply those ideas to any D dimensional data. But of course you cannot visualize D dimensional data anymore, and you always have to be careful of what happens if you expand degree of dimension.

Conversely it is also important to reduce dimension to understand abstract high dimensional stuff in 2 or 3 dimensional space, which are close to our everyday sense. That means dimension reduction is one powerful way of data visualization.

In this blog series I am going to explain meanings of dimension itself in machine learning context and algorithms for dimension reductions, such as PCA, LDA, and t-SNE, with 2 or 3 dimensional visible data. Along with that, I am going to delve into the meaning of calculations so that you can understand them in more like everyday-life sense.

#### This article series is going to be roughly divided into the contents below.

1. Curse of Dimensionality
2. Rethinking linear algebra: visualizing linear transformations and eigen vector
3. The algorithm known as PCA and my taxonomy of linear dimension reductions
4. Rethinking linear algebra part two: ellipsoids in data science
5. Autoencoder as dimension reduction (to be published soon)
6. t-SNE (to be published soon)

I hope you could see that reducing dimension is one of the fundamental approaches in data science or machine learning.

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