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Handling constraints in NDSolve


Dear Folks,

I am not sure how to use NDSolve with an equation set that contains 
constraints. A very simple example is shown below.  There are two 
unknowns (x and y) which are functions of t. I have two equations and 
impose two initial conditions. Seems like it should work, but I am 
probably doing something foolish. If I remove the constraints, the 
system works fine. Any advice?


Clear[x, y, t];
T = 1200;                            
Eact = 149000;                   
kzero = 5.468*10^6;
rho0 = 1.834;
Rgas = 8.314;
L = 1;
k1 = kzero* Exp[-Eact/(Rgas*T)];

s = NDSolve[
        { x'[t] == x[t](L - x[t])*(1 - y[t]),
         y'[t] == -k1*y[t] /; t < x[t]/x'[t],
         y'[t] == 0 /; t =84 x[t]/x'[t],
         x[0] == 0.001,
         y[0] == 1},
       {y[t], x[t]}, {t, 0, 100.}
];
y1a = y[t] /. s[[1]][[1]];
xa = x[t] /. s[[1]][[2]]


Plot[y1a, {t, 0, 20}, PlotRange -> All];
Plot[xa, {t, 0.001, 20}, PlotRange -> All];


Thanks for any help you can give me.

Phil
-- 
Philip M. Howe
Program Manager, Stockpile Surety
Los Alamos National Laboratory

(505) 665-5332
(505) 667-9498
Fax: 505-665-5249
email pmhowe at lanl.gov
Mail Stop F606


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