YES(?,ELEMENTARY) We are left with following problem, upon which TcT provides the certificate YES(?,ELEMENTARY). Strict Trs: { f(0()) -> s(0()) , f(s(0())) -> s(s(0())) , f(s(0())) -> *(s(s(0())), f(0())) , f(+(x, y)) -> *(f(x), f(y)) , f(+(x, s(0()))) -> +(s(s(0())), f(x)) } Obligation: innermost runtime complexity Answer: YES(?,ELEMENTARY) The input was oriented with the instance of 'Lightweight Multiset Path Order' as induced by the safe mapping safe(f) = {}, safe(0) = {}, safe(s) = {1}, safe(*) = {1, 2}, safe(+) = {1, 2} and precedence empty . Following symbols are considered recursive: {f} The recursion depth is 1. For your convenience, here are the satisfied ordering constraints: f(0();) > s(; 0()) f(s(; 0());) > s(; s(; 0())) f(s(; 0());) > *(; s(; s(; 0())), f(0();)) f(+(; x, y);) > *(; f(x;), f(y;)) f(+(; x, s(; 0()));) > +(; s(; s(; 0())), f(x;)) Hurray, we answered YES(?,ELEMENTARY)