A congruence modulo $n^3$ involving two consecutive sums of powers and its applications

For various positive integers $k$, the sums of $k$th powers of the first $n$ positive integers, $S_k(n+1)=1^k+2^k+\mathrm{.}\mathrm{.}\mathrm{.}+n^k$, have got to be some of the most popular sums in all of mathematics. In this note we prove that for each $k\ge 2$$ 2S_{2k+1}(n)- (2k+1)nS_{2k}(n)\equiv {{array}{ll} 0,(\bmod{,n^3}) & {\rm if},,k,,{\rm is,,even,,or},, n,, {\rm is,, odd} & {\rm or} ,, n\equiv 0,(\bmod{,4}) \frac{n^3}{2},(\bmod{,n^3}) & {\rm if},,k,,{\rm is,, odd} &,,{\rm and},, n\equiv 2,(\bmod{,4}). {array}.$$ The above congruence allows us to state an equivalent formulation of Giuga’s conjecture. Moreover, we prove that the first above congruence is satisfied modulo $n^4$ whenever $n\ge 5$ is a prime number such that $n-1\nmid 2k-2$. In particular, this congruence arises a conjecture for a prime to be Wolstenholme prime. We also propose several Giuga-Agoh’s-like conjectures. Further, we establish two congruences modulo $n^3$ for two binomial type sums involving sums of powers $S_{2i}(n)$ with $i=0,1,\mathrm{.}\mathrm{.}\mathrm{.},k$. Furthermore, using the above congruence reduced modulo $n^2$, we obtain an extension of Carlitz-von Staudt result for odd power sums.