“…Arguably, the most significant open problem in the theory concerns the maximal monotonicity of the sum of two maximally monotone operators in general Banach spaces; this is called the "sum problem". Some recent developments on the sum problem can be found in Simons' monograph [28] and [4,5,6,8,12,11,36,19,31,37,38,39]. It is known, among other things, that the sum theorem holds under Rockafellar's constraint qualification when both operators are of dense type or when each operator has nonempty domain interior [8,Ch.…”