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(First of all I never saw Hilbert spaces in a mathematical class, only used it in intro QM so far, so please don't assume I know that much when answering.)
Let's consider the Hilbert space on the interval [a,b] and the operator ##\textbf{L} = \frac{d^{2}}{dx^{2}} ##. Then ##\textbf{L}## is hermitic only if for all functions ##f(x)## and ##g(x)## in the Hilbert space:
##<f| \textbf{L} g > = < \textbf{L} f |g > ##
Using partial integration one can find that:
##<f| \textbf{L} g > - < \textbf{L} f |g > = g'(b)f_{c}(b)-g'(a)f_{c}(a)-f_{c}'(b)g(b)+f_{c}'(a)g(a)##
( ' refers to the first derivative and subscript c to the complex conjugate )
In physics class now something weird happened. We said that the functions we will be working with will have certain boundary conditions we had discussed such that the right term in the last equation vanishes. And thus in our case the operator ##\textbf{L}## will be hermitian. This means we will use the eigenfunctions of ##\textbf{L}## as a basis for our space, saying that they can produce any square integrable function on [a,b].
This confuses me A LOT. I thought that the relationship above had to hold for ALL square integrable functions ##\textbf{L}## and only then the eigenfunctions could produce all the square integrable functions on [a,b].
So basically something fishy is happening in physics class and I'd like it explained on a non-math level since I have not taken a math course on Hilbert spaces and only have practical experiences of working with them in intro QM.
Let's consider the Hilbert space on the interval [a,b] and the operator ##\textbf{L} = \frac{d^{2}}{dx^{2}} ##. Then ##\textbf{L}## is hermitic only if for all functions ##f(x)## and ##g(x)## in the Hilbert space:
##<f| \textbf{L} g > = < \textbf{L} f |g > ##
Using partial integration one can find that:
##<f| \textbf{L} g > - < \textbf{L} f |g > = g'(b)f_{c}(b)-g'(a)f_{c}(a)-f_{c}'(b)g(b)+f_{c}'(a)g(a)##
( ' refers to the first derivative and subscript c to the complex conjugate )
In physics class now something weird happened. We said that the functions we will be working with will have certain boundary conditions we had discussed such that the right term in the last equation vanishes. And thus in our case the operator ##\textbf{L}## will be hermitian. This means we will use the eigenfunctions of ##\textbf{L}## as a basis for our space, saying that they can produce any square integrable function on [a,b].
This confuses me A LOT. I thought that the relationship above had to hold for ALL square integrable functions ##\textbf{L}## and only then the eigenfunctions could produce all the square integrable functions on [a,b].
So basically something fishy is happening in physics class and I'd like it explained on a non-math level since I have not taken a math course on Hilbert spaces and only have practical experiences of working with them in intro QM.
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