Results of running Nordhaus with unconstrained investment (1: s be free; 2: delete Nordhaus’ terminal condition on k and investment; 3: let investment negative but keep end-of-period k nonnegative) using other solvers:
Knitro-IP: got a good solution (but the first 2 runs failed due to (1) Iteration limit reached; (2) FUNC DOMAIN: x**c, x < 0 in equation YY(1) and ABATEEQ(1))
Knitro-3: Failed in getting solution: FUNC DOMAIN: x**c, x < 0 in equation YY(2)
SNOPT: got a solution which is good before 40 decades.
IPOPT: solution is good, almost the same with CONOPT
MINOS: got a solution which is good before 38 decades.