Хабрахабр

# How linear algebra is applied in machine learning

How are you going to apply all this inversions, transpositions, eigenvector and eigenvalues for practical purposes? When you study an abstract subject like linear algebra, you may wonder: why do you need all these vectors and matrices?

Well, if you study linear algebra with the purpose of doing machine learning, this is the answer for you.

In brief, you can use linear algebra for machine learning on 3 different levels:

• application of a model to data;
• training the model;
• understanding how it works or why it does not work. If not, maybe now it's a good time to read about them in Wikipedia, or even signup for a MOOC on these subjects. I assume that you, the reader, have at least a vague idea of linear algebra concepts (such as vectors, matrices, their products, inverse matrices, eigenvectors and eigenvalues), and machine learning problems (such as regression, classification and dimensionality reduction).

### Application

When we have this representation , we can use it e.g. What machine learning usually does is fitting some function , where is the input data, is some useful representation of this data, and are additional parameters, on which our function depends and which have to be learned. to reconstruct the original data (as in unsupervised learning), or to predict some value of interest, (as in supervised learning).

But storage alone is not important. All of , , and are usually numeric arrays, and can be at least stored as vectors and matrices. Examples of such linear algorithms are: The important thing is that our function is often linear, that is, .

A simple neural network may look like — it uses two matrix multiplications, and a nonlinear transformation between them. Other algorithms, like neural network, learn nonlinear transformations, but still rely heavily on linear operations (that is, matrix-matrix or matrix-vector multiplication).

### Training

The loss itself is sometimes convenient to write in terms of linear algebra. To train an algorithm, you usually define a loss function and try to optimize it. This representation is useful, because it enables us to derive ways to minimize this loss. For example, the quadratic loss (used in the least squares method) can be written as a dot product , where is the vector of your prediction, and is the ground truth you try to predict. Lots of linear operations in one place! For example, if you use linear regression with this least squares method, then your optimal solution looks like .

Another example of linear solution is PCA, where the parameters of interest are the first eigenvectors of the matrix , corresponding to the largest eigenvalues.

To do this, you need to differentiate the loss w.r.t. If you train neural networks, there is usually no analytical solution for the optimal parameters, and you have to use gradient descent. the parameters, and it while doing so, you again have to multiply matrices, because if (a composite function), then , and all these derivatives are matrices or vectors, because and are multidimensional.

You can speed it up, by applying Newtonian optimization methods. Simple gradient descent is OK, but it is slow. the parameters . The basic method is , where and are are the vector of first derivatives and the matrix of the second derivatives of your loss w.r.t. But it can be unstable and/or computationally expensive, and you may need to come up with its approximations (like L-BFGS) that use even more involved linear algebra for quick and cheap optimization.

### Analysis

But the real science (or magic) starts when your model refuses to train or predict well. You see that linear algebra helps you to apply and to train your models. In deep learning, it often happens due to vanishing or exploding gradients. The learning may get stuck at a bad point, or suddenly go wild. One of the ways to inspect what is happening is to keep track of the eigenvalues of the matrices you are trying to invert. That is, whey you calculate the gradient, you multiply lots of matrices, and then strange things happen, and you need to know what, why, and how to overcome it. If you multiply many matrices with large eigenvalues, the product explodes. If they are close to 0, or just very different, then the inversion of this matrix can lead to unstable results. When these eigenvalues are small, the result fades to zero.

If you want to apply any of those techniques, you need a way to measure whether they help much for your particular problem. Different techniques, like L1/L2 regularization, batch normalization, and LSTM were invented in order to fight these problems with convergence. This again involves lots of manipulation with vectors, matrices, their decompositions, etc. And if you want to invent such a technique yourself, you need a way to prove that it can work at all.

### Conclusion

To apply pre-trained models, you have to at least convert your data into a format, compatible with linear algebra (e.g. You can see that the deeper you dive into machine learning, the more linear algebra you see there. If you need to implement a training algorithm, or even to invent a new one, be prepared to multiply, invert, and decompose lots of matrices. numpy.array in Python).

It's okay. In this text, I have referenced some concepts you may be unfamiliar with. What this article encourages you is to search for the unknown words and enlarge your horizons.

By the way, it would be interesting to hear some stories from you in the comments, about how you encountered applications of linear algebra in your own job or study.

S. P. However, I never said that maths is useless (otherwise, I wouldn't be teaching it all the time). In one of my articles, I argued that you don't have to learn maths in order to become successful (it's still a popular stereotype in Russia), even if you work in IT. Usually it is not the key to success, but in many cases, it helps, and in a few (like developing deep learning models), it is essential.

P. P. Почему на английском?! Ну просто потому что могу. S. А потом решил, что ответ можно довести до уровня маленькой публичной статьюшки. Оригинальный вопрос мне задали на этом языке, и на английском же я на него ответил.

Во-вторых, Хабр вроде сам собирался выходить на международные рынки — так почему бы не попробовать разместить тут кусочек англоязычного контента? Почему тогда Хабр, а не, например, Медиум? Во-первых, в отличие от Медиума, тут нормально поддержаны формулы.

Посмотрим, что из этого получится.

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