Troubles with NDSolve--missing arguments of function

*To*: mathgroup at smc.vnet.net*Subject*: [mg81041] Troubles with NDSolve--missing arguments of function*From*: "lederer at ssb.rochester.edu" <lederer at ssb.rochester.edu>*Date*: Sun, 9 Sep 2007 06:11:14 -0400 (EDT)

I am having a problem with NDSolve. I am trying to solve a two function- calculus of variations problem by solving a system of differential equations. Here is a simplified description of my current troubles, reducing the equations for the problem to one function, b[x], b'[x], defined on a closed interval on the real line. When applying NDSolve, Mathematica keeps giving me a message saying that =E2=80=9Cb=E2=80=9D --- is missing its parameter. This is where I think it is coming from: As one of constraints I have something like: =E2=88=82f[b,b=E2=80=99[x],x]/=E2=88=82b * db[x]/dx==0 with the expression evaluated at b=b[x] that is, my constraint is: the partial derivative of the function f=E2=80=99s first argument evaluated at b[x] and then multiplied by the derivative of b[x] wrt x=E2=80=A6.. and is set to zero. The way I am doing this in Mathematica is ( D[f[b,b'[x], x],b]/.b->b[x] )*D[b[x],x]==0 I think the program is upset that the left hand side of one of the constraints has a function =E2=80=9Cb=E2=80=9D without its usual argument, b[x] But if I try: D[ f[ b[x],b'[x],x] , x ] I will get the complete derivative of f wrt x which is just plain wrong. Two related questions: How can I get the constraint I want? and/or Is what I am doing correct, but my diagnosis of the error message is (likely) wrong? Many thanks in advance. Phil