# Thread:

1. ## Functional Analysis question

I can't work this out

show $c_0$ (with the usual sup norm) is not a Hilbert Space.

my main problem stems from using the $c_0$ space which I don't fully grasp, I know the method for this type of problem so I'm really looking for a suggestion as a what to let x and y equal and what their norms should look like

2. Originally Posted by blimp
I can't work this out

show $c_0$ (with the usual sup norm) is not a Hilbert Space.

my main problem stems from using the $c_0$ space which I don't fully grasp, I know the method for this type of problem so I'm really looking for a suggestion as a what to let x and y equal and what their norms should look like
The way to show results like this is to use the parallelogram identity. You can choose almost any two elements of the space to see that they do not satisfy the identity. The easiest choice would be to take for example x to be the sequence in $c_0$ having a 1 for its first coordinate and 0 for every other coordinate; and take y to be the sequence having a 1 for its second coordinate and 0 for every other coordinate. Then x, y, x+y and x–y all have $c_0$-norm 1
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