a 2 + b 2 = c 2 {\displaystyle a^{2}+b^{2}=c^{2}} ∑ m = 1 ∞ ∑ n = 2 ∞ m 2 n 3 m ( m 3 n + n 3 m ) {\displaystyle \sum _{m=1}^{\infty }\sum _{n=2}^{\infty }{\frac {m^{2}\,n}{3^{m}\left(m\,3^{n}+n\,3^{m}\right)}}}
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