Getting number out of NDSolve

*To*: mathgroup at smc.vnet.net*Subject*: [mg24404] Getting number out of NDSolve*From*: colton at socrates.berkeley.edu (John S. Colton)*Date*: Tue, 18 Jul 2000 00:58:04 -0400 (EDT)*Organization*: University of California at Berkeley*Sender*: owner-wri-mathgroup at wolfram.com

Hi, all, I used Mathematica's function NDSolve to solve some coupled differential equations. Following the example in the manual, the command was something like this: sol = NDSolve[{ x1''[t] + eta x1'[t] + 2 Pi Sin[2 Pi x1[t]] - k (x2[t]-x1[t]-a12[t]) == 0, x2''[t] + eta x2'[t] + 2 Pi Sin[2 Pi x2[t]] - k (x3[t]-x2[t]-a23[t]) + k (x2[t]-x1[t]-a12[t]) == 0, x3''[t] + eta x3'[t] + 2 Pi Sin[2 Pi x3[t]] + k (x3[t]-x2[t]-a23[t]) == 0, x1[0] == 0, x2[0] == 1, x3[0] == 2, x1'[0] == x2'[0] == x3'[0] == 0}, {x1,x2,x3},{t,0,tmax}] (a12 and a23 functions were defined earlier, as well as the eta and k constants) Then, to plot the solutions, I used something like this: Plot[{Evaluate[x1[t] /. sol],Evaluate[x2[t] /. sol],Evaluate[x3[t] /. sol]}, {t,0,tmax}] Worked just great! Solutions looked like what I expected. However, when attempting to get a number for the functions x1,x2, or x3, for a particular time, all my attempts fail short. I've tried things like N[x1[tmax]], or Evaluate[x1[tmax] /. sol, or Evaluate[x1[t] /. sol /. t -> tmax, etc,etc. I can't get anything to spit out a number. I appreciate any help. John