A proof of all ranks S-duality conjecture for K3 surfaces

Abstract: Using the multiple cover formula of Y. Toda for counting invariants of semistable twisted sheaves over twisted local K3 surfaces we calculate the $\SU(r)/\zz_r$-Vafa-Witten invariants for K3 surfaces for any rank $r$ for the Langlands dual group $\SU(r)/\zz_r$ of the gauge group $\SU(r)$. We generalize and prove the S-duality conjecture of Vafa-Witten for K3 surfaces in any rank $r$ based on the result of Tanaka-Thomas for the $\SU(r)$-Vafa-Witten invariants.