What Is An Eigenvector?

[rohit-lotlikar-b6b3af34 (Rohit Lotlikar)]

I would like to explain this concept in a way that we can easily understand it.

For the sake of simplicity, let’s consider that we live in a two-dimensional world.

• Alex’s house is located at coordinates [10,10] (x=10 and y =10). Let’s refer to it as vector A.
• Furthermore, his friend Bob lives in a house with coordinates [20,20] (x=20 and y=20). I will refer to it as vector B.

If Alex wants to meet Bob at his place then Alex would have to travel +10 points on the x-axis and +10 points on the y-axis. This movement and direction can be represented as a two-dimensional vector [10,10]. Let’s refer to it as vector C.
We can see that vector A to B are related because vector B can be achieved by scaling (multiplying) the vector A by 2. This is because 2 x [10,10] = [20,20]. This is the address of Bob. Vector C also represents the movement for A to reach B.
The key to note is that a vector can contain the magnitude and direction of a movement. So far so good!
We learned from the introduction above that large set of data can be represented as a matrix and we need to somehow compress the columns of the sparse matrix to speed up our calculations. Plus if we multiply a matrix by a vector then we achieve a new vector. The multiplication of a matrix by a vector is known as transformation matrices.
We can transform and change matrices into new vectors by multiplying a matrix with a vector. The multiplication of the matrix by a vector computes a new vector. This is the transformed vector. Hold that thought for now!
The new vector can be considered to be in two forms:

1. Sometimes, the new transformed vector is just a scaled form of the original vector. This means that the new vector can be re-calculated by simply multiplying a scalar (number) to the original vector; just as in the example of vector A and B above.
2. And other times, the transformed vector has no direct scalar relationship with the original vector which we used to multiply to the matrix.
   If the new transformed vector is just a scaled form of the original vector then the original vector is known to be an eigenvector of the original matrix. Vectors that have this characteristic are special vectors and they are known as eigenvectors. Eigenvectors can be used to represent a large dimensional matrix.
   Therefore, if our input is a large sparse matrix M then we can find a vector o that can replace the matrix M. The criteria is that the product of matrix M and vector o should be the product of vector o and a scalar n:
   M * o = n* o
   This means that a matrix M and a vector o can be replaced by a scalar n and a vector o.
   In this instance, o is the eigenvector and n is the eigenvalue and our target is to find o and n.
   Therefore an eigenvector is a vector that does not change when a transformation is applied to it, except that it becomes a scaled version of the original vector.
   Eigenvectors can help us calculating an approximation of a large matrix as a smaller vector. There are many other uses which I will explain later on in the article.
   Eigenvectors are used to make linear transformation understandable. Think of eigenvectors as stretching/compressing an X-Y line chart without changing their direction.