       bvdae error when using NDSolve for initial value problem

• To: mathgroup at smc.vnet.net
• Subject: [mg102063] bvdae error when using NDSolve for initial value problem
• From: Damon <damonwischik at gmail.com>
• Date: Tue, 28 Jul 2009 02:05:14 -0400 (EDT)

```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,
w1==1.5,
w2==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,
w1==1.5,
w2==1,
timer==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.

Damon.

```

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