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Questions about the properties of vector spaces and linear transformations, including linear systems in general.

27 votes

The number $\pi$ and summation by $SL(2,\mathbb Z)$

I add our explanation and the origin of the problem. To obtain the formula we just need to verify the following lemma (by a straightforward computation). Magic Lemma. Let $(a,b),(c,d)$ be as in …
Nikita Kalinin's user avatar
115 votes
3 answers
5k views

The number $\pi$ and summation by $SL(2,\mathbb Z)$

Let $f(a,b,c,d)=\sqrt{a^2+b^2}+\sqrt{c^2+d^2}-\sqrt{(a+c)^2+(b+d)^2}$. (it is the defect in the triangle inequality) Then, we discovered by heuristic arguments and then verified by computer that $$\ …
Nikita Kalinin's user avatar