YES(?,O(n^3)) We are left with following problem, upon which TcT provides the certificate YES(?,O(n^3)). Strict Trs: { from(X) -> cons(X, n__from(s(X))) , from(X) -> n__from(X) , after(s(N), cons(X, XS)) -> after(N, activate(XS)) , after(0(), XS) -> XS , activate(X) -> X , activate(n__from(X)) -> from(X) } Obligation: innermost runtime complexity Answer: YES(?,O(n^3)) The input was oriented with the instance of 'Small Polynomial Path Order (PS)' as induced by the safe mapping safe(from) = {1}, safe(cons) = {1, 2}, safe(n__from) = {1}, safe(s) = {1}, safe(after) = {2}, safe(0) = {}, safe(activate) = {1} and precedence after > from, after > activate, activate > from . Following symbols are considered recursive: {from, after, activate} The recursion depth is 3. For your convenience, here are the satisfied ordering constraints: from(; X) > cons(; X, n__from(; s(; X))) from(; X) > n__from(; X) after(s(; N); cons(; X, XS)) > after(N; activate(; XS)) after(0(); XS) > XS activate(; X) > X activate(; n__from(; X)) > from(; X) Hurray, we answered YES(?,O(n^3))