exo7 4490

$\sum_{(i,j)\in(\N^*)^2}\frac1{(i+j)^\alpha}$.

$\sum_{(i,j)\in(\N^*)^2}\frac1{\strut i^\alpha +j^\alpha}$.

$\sum_{x\in\Q\cap[1,+\infty[}\frac1{x^2}$.

$\sum_{(p,q)\in\N^2}\frac1{a^p+b^q}$, ${a>1,b>1}$.