Elena Celledoni Arne Marthinsen Brynjulf Owren Commutator-free Lie group methods. 341-352 2003 19 Future Generation Comp. Syst. 3 http://dx.doi.org/10.1016/S0167-739X(02)00161-9 db/journals/fgcs/fgcs19.html#CelledoniMO03