Foreword
An interesting example of infinite dimensional Hilbert space is given by the set of sequences $x=(x_1,x_2,...x_k,.....)$, being $x_k$ complex numbers.
This set can be structured as Vector Space putting :
$lambda*x = (lambda*x_1,lambda*x_2,..., lambda*x_k,....) $ ;$ x+y = (x_1+y_1,x_2+y_2,....x_k+y_k,....)$
The subset $l^2 $ of the sequences which verify the condition :
$sum_(k>=1) |x_k|^2 < +oo $
is a Vector Subspace of the previous one ; the scalar product can be defined as follows :
$<x,y> = sum_(k>=1) x_ky_k^* $ being $ y_k^* $ the coniugate of $y_k $.
Let's try this exercise :
In the $l^2$ space of real sequences $x =(x_n )_(n=1)^oo $ such that $||x||_2 = sqrt(sum|x_n|^2) < +oo $ let's consider the infinite dimensional vectors :
$x=(2,0,0,.......) ; y = (1/k)_(k=1)^oo $
a) Calculate the angle formed by the vectors $ x $ and $y $ .
b) Orthormalize the sistem $(x,y) $ that is find an orthonormal system that generates the same space as $(x,y) $.