f ( x ) = x 2 {\displaystyle f(x)=x^{2}}
∑ m = 1 ∞ ∑ n = 1 ∞ m 2 n 3 m ( m 3 n + n 3 m ) {\displaystyle \sum _{m=1}^{\infty }\sum _{n=1}^{\infty }{\frac {m^{2}n}{3^{m}\left(m3^{n}+n3^{m}\right)}}}
