Re: bvdae error when using NDSolve for initial value problem

• To: mathgroup at smc.vnet.net
• Subject: [mg102123] Re: bvdae error when using NDSolve for initial value problem
• From: Roland Franzius <roland.franzius at uos.de>
• Date: Thu, 30 Jul 2009 05:31:10 -0400 (EDT)
• References: <h4m4ec\$ef0\$1@smc.vnet.net>

```Damon schrieb:
> I am trying to solve an initial value problem. I am getting the error
> message "bvdae: Differential-algebraic equations must be given as
> initial value problems". I don't know what the error means or how to
> fix my equations. I would be grateful for any help.
>
> I started with the following differential equations. I want to find
> the time T when timer[T]==8.3 (or some other arbitrary value), and I
> want to evaluate w1[T] and w2[T] at that time.
>
> res = NDSolve[{
> w1'[u]==w1[u] 1/w[u] (w1[u]/w[u])^(1-\[Epsilon]),
> w2'[u]==w2[u] 1/w[u] (w2[u]/w[u])^(1-\[Epsilon]),
> w[u]==w1[u]+w2[u],
> timer'[u]==(p1 w1[u]+p2 w2[u]),
> timer[0]==0,
> w1[0]==1.5,
> w2[0]==1
> } /. {p1->.01,p2->.01,\[Epsilon]->.5},
> {w1,w2,w,timer},{u,0,11}
> ]
>
> I transformed this into an initial value problem in standard form as
> follows.
>
> Clear[w1,w2,timer];
> res = NDSolve[{
> w1'[u]==T[u] w1[u] 1/w[u] (w1[u]/w[u])^(1-\[Epsilon]),
> w2'[u]==T[u] w2[u] 1/w[u] (w2[u]/w[u])^(1-\[Epsilon]),
> w[u]==w1[u]+w2[u],
> T'[u]==0,
> timer'[u]==T[u] (p1 w1[u]+p2 w2[u]),
> timer[0]==0,
> w1[0]==1.5,
> w2[0]==1,
> timer[1]==8.3
> } /. {p1->.01,p2->.01,\[Epsilon]->.5},
> {w1,w2,w,T,timer},{u,0,1}
> ]
>
> This gives me the error I quoted at the top of this post. I would be
> grateful for hints about what I should do to solve my problem.

You cannot set intermediate values like that for timer[1] in an initial
value problem for a system of ODE's.

--

Roland Franzius

```

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