Evaluate:
$$\sum_{k=1}^\infty\left(\zeta(2)-\sum_{n=1}^k\frac1{n^2}\right)^2$$
Recognizing that $\zeta(2)-\sum_{n=1}^k\frac1{n^2}$ can be written as $\psi_1(1+k)$ where $\psi_1(z)$ is the trigamma function,
What remains to be done is to evaluate:
$$\sum_{k=1}^\infty\psi_1^2(k+1)$$
Mathematica could not evaluate it in a closed form but the source assures that it exists.
If you liked this problem check out Hard Definite integral involving the Zeta function.
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