NDSolve problem

*To*: mathgroup at christensen.cybernetics.net*Subject*: [mg900] NDSolve problem*From*: pitts at mayo.EDU (Todd Pitts)*Date*: Fri, 28 Apr 1995 00:58:01 -0400*Organization*: Ultrasound Research Laboratory, Mayo Foundation, Rochester MN

I have been using NDSolve[] to investigate the solutions to some large, sets of simple, ordinary differential equations. It is most convenient to use subscripted variables/functions and parameters to represent the large number of solution functions and system characteristics. For certain values of coefficients the solution proceeds nicely, however, for others I obtain the error NDSolve::ndnum: Differential equation does not evaluate to a number at t = 0.. Below I have described the simplest system that I have been able to obtain an error on. (* System equation *) eq={4.0*D[x[1][t],{t,2}]==x[1][t]-Sin[t]}; (* Initial conditions *) bc={Evaluate[D[x[1][t],t] /. t->0]==0,x[1][0]==0}; (* Make a list of all system equations *) sys=Join[eq,bc]; (* Send the problem to NDSolve[] *) NDSolve[sys,x[1],{t,0,10}] Yields the following output: NDSolve::ndnum: Differential equation does not evaluate to a number at t = 0.. Out[32]= {{x[1] -> InterpolatingFunction[{0., 0.}, <>]}} Changing the coefficient 4.0 in eq to unity (1 not 1.0) gives no error. Also writing the equation as eq={D[x[1][t],{t,2}]==(x[1][t]-Sin[t])/4}; works. If I don't use subscripted variables I don't have this problem. For example sys2 = sys /. x[1] -> x NDSolve[sys2,x,{t,0,10}] seems to work fine. I would greatly appreciate any hints. Is this a bug or have I misunderstood the use of subscripted variables? The version number is 2.2. I get the same behavior on NeXT and DEC Alfa. Thanks in Advance, Todd Pitts