Some evaluation of cubic Euler sums

Abstract

P. Flajolet and B. Salvy \cite{FS1998} prove the famous theorem that a nonlinear Euler sum $S_{i_1i_2\cdots i_r,q}$ reduces to a combination of sums of lower orders whenever the weight $i_1+i_2+\cdots+i_r+q$ and the order $r$ are of the same parity. In this article, we develop an approach to evaluate the cubic sums $S_{1^2m,p}$ and $S_{1l_1l_2,l_3}$. By using the approach, we establish some relations involving cubic, quadratic and linear Euler sums. Specially, we prove the cubic sums $S_{1^2m,m}$ and $S_{1(2l+1)^2,2l+1}$ are reducible to zeta values, quadratic and linear sums. Moreover, we prove that the two combined sums involving multiple zeta values of depth four \[\sum\limits_{\left\{ {i,j} \right\} \in \left\{ {1,2} \right\},i \ne j} {ζ\left( {{m_i},{m_j},1,1} \right)}\quad {\rm and}\quad \sum\limits_{\left\{ {i,j,k} \right\} \in \left\{ {1,2,3} \right\},i \ne j \ne k} {ζ\left( {{m_i},{m_j},{m_k},1} \right)} \] can be expressed in terms of multiple zeta values of depth $\leq 3$, here $2\leq m_1,m_2,m_3\in \N$. Finally, we evaluate the alternating cubic Euler sums ${S_{{{\bar 1}^3},2r + 1}}$ and show that it are reducible to alternating quadratic and linear Euler sums. The approach is based on Tornheim type series computations.