$32$ Goldbach Variations - Papers presenting a single gem in number theory or combinatorics from different point of view A short time ago I found the nice paper Thirty-two Goldbach Variations written by J.M. Borwein and D.M. Bradley. It presents $32$ different proofs of the Euler sum identity
\begin{align*}
\zeta(2,1)=\zeta(3)=8\zeta(\overline{2},1)
\end{align*}

Such a walk through different methods around a single theme is for me an extraordinary pleasure and a good opportunity to associate and to link aspects which I wasn't aware before.
Here I'm asking for papers like this one which present a single gem and  provide us with an anthology of different representions, or different proofs or other aspects around this gem. To keep the selection managable I'd like to put the focus on number theory and combinatorics.

Two more examples which perfectly match my requirements/wishes:


*

*Catalan Addendum by R.P. Stanley provides us with a wealth of different representations of Catalan Numbers extending his $66$ combinatorial representations from the second volume of his classic Enumerative Combinatorics.

*The Harmonic Series Diverges Again and Again by S.J. Kifowit and T.A. Stamps
presenting some rather elementary proofs together with the follow-up paper More Proofs of Divergence of the Harmonic Series.

Robin Chapman gives many proofs of $\sum n^{-2}=\pi^2/6$. 
