Talk:Square-integrable function

The article says, "The quadratically integrable functions form an inner product space." I'm pretty sure that isn't true! A function that differs from the zero function on a non-empty set of measure zero will have inner product zero, violating positive-definiteness. That's why you have to mod out by these functions when you define L^p spaces, right? ---Vectornaut (talk) 01:44, 29 May 2011 (UTC)[reply]

Two functions may attain different values on a set of measure zero and still be considered equal under any $L_{p}$ metric. Only the $L_{2}$ metric is induced by an inner product, because the polarization identity does not hold for other values of $p$ . Therefore the other $L_{p}$ spaces are not inner product spaces. The equality of functions in the context of Lebesgue integration is redefined in terms of the Lebesgue integral of their difference. Therefore, even if they do differ on a set of Lebesgue measure zero, they are equal under the revised equality. This is how an $L_{p}$ space can be a metric space and how an $L_{2}$ space is an inner product space. So the condition of sign definiteness (or positive definiteness, although that is redundant and can be derived by sign definiteness and the triangle inequality) holds for the $L_{p}$ metrics under the revised equality. For more details see Naylor and Sell, Linear Operator Theory in Engineering and Science, Springer. — Preceding unsigned comment added by Helptry (talk • contribs) 03:21, 29 November 2012 (UTC)[reply]

@Helptry What you write is blatently wrong (very unusual at least). Two functions are equal, if their graphs are equal. What happens here is that the vector space N of Null-functions (functions whose support is a measure-0-set) must be (is) factored out from the space of sq-integrable functions cL2 (curlyL). Thus L2=cL2/N. The vector space of classes f+N, f sq. int. (f in cL2), then becomes a Hilbert space. This is correctly described in the article about L2 on WP. What you describe as "new equality" is equality in the quotient space cL2/N. — Preceding unsigned comment added by LMSchmitt (talk • contribs) 19:53, 24 November 2021 (UTC)[reply]

Why does L2 norm redirect here!?

I'm looking for an explanation of L2 norm. I get here instead and I see L2 and I see norm, but if you are going to redirect me here then I would like to see L2 norm. — Preceding unsigned comment added by 108.80.114.110 (talk) 11:42, 10 July 2014 (UTC)[reply]

For what it's worth, L2 norm redirects to Norm (mathematics)#Euclidean norm, and L2-norm redirects to Square-integrable function. I think L2-norm should be fixed to redirect to Norm (mathematics)#Euclidean norm, too. – Tobias Bergemann (talk) 11:57, 10 July 2014 (UTC)[reply]

"L2 space" listed at Redirects for discussion

A discussion is taking place to address the redirect L2 space. The discussion will occur at Wikipedia:Redirects for discussion/Log/2020 June 10#L2 space until a consensus is reached, and readers of this page are welcome to contribute to the discussion. 1234qwer1234qwer4 (talk) 16:58, 10 June 2020 (UTC)[reply]