Another idea. >From your formulation it seems that r does not depend on t and Xt so you wouldn't need a solver. You could solve for r explicitly so r <- R0/sum(Xt) /Berend -- View this message in context: http://r.789695.n4.nabble.com/Solver-in-R-tp2535115p2535815.html Sent from the R help mailing list archive at Nabble.com.