Re: NDSolve

*To*: mathgroup at smc.vnet.net*Subject*: [mg65687] Re: NDSolve*From*: bghiggins at ucdavis.edu*Date*: Sun, 16 Apr 2006 01:44:44 -0400 (EDT)*References*: <e1nnch$lnr$1@smc.vnet.net>*Sender*: owner-wri-mathgroup at wolfram.com

You can evaluate the derivate from the solution in several ways. The first way (not the most convenient) is sol1 = NDSolve[{y''[x] == Cos[y[x]], y[0] == 1, y'[0] == 0}, y[x], {x, 1, 10}] {{y[x] -> InterpolatingFunction[{{1., 10.}}, <>][x]}} Then to determine the derivative at a point D[y[x]/.sol1,x]/.x->2. {0.519544} If you are interested in the value of the derivative and plotting the derivative (i.e a phase plot), then it is more convienet to get the rule for the solution in the form y->InterpolationFunction rather than y[x]->InterpolationFunction as follows: sol2 = NDSolve[{y''[x] == Cos[y[x]], y[0] == 1, y'[0] == 0}, y, {x, 1, 10}] {{y -> InterpolatingFunction[{{1., 10.}}, <>]}} Then to find the derivative we simply evaluate y'[2.]/.sol2 {0.519544} Hope this helps, Cheers, Brian