### abstract

- The \(S\)-functional calculus is a functional calculus for \((n+1)\)-tuples of not necessarily commuting operators that can be considered a higher-dimensional version of the classical Rieszâ€“Dunford functional calculus for a single operator. In this last calculus, the resolvent equation plays an important role in the proof of several results. Associated with the \(S\)-functional calculus there are two resolvent operators: the left \(S_L^{-1}(s,T)\) and the right one \(S_R^{-1}(s,T)\), where \(s=(s_0,s_1,\ldots ,s_n)\in \mathbb {R}^{n+1}\) and \(T=(T_0,T_1,\ldots ,T_n)\) is an \((n+1)\)-tuple of noncommuting operators. The two \(S\)-resolvent operators satisfy the \(S\)-resolvent equations \(S_L^{-1}(s,T)s-TS_L^{-1}(s,T)=\mathcal {I}\), and \(sS_R^{-1}(s,T)-S_R^{-1}(s,T)T=\mathcal {I}\), respectively, where \(\mathcal {I}\) denotes the identity operator. These equations allow us to prove some properties of the \(S\)-functional calculus. In this paper we prove a new resolvent equation which is the analog of the classical resolvent equation. It is interesting to note that the equation involves both the left and the right \(S\)-resolvent operators simultaneously.