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stepwise ode. why does this fail?


Why does this fail? I don't understand. I think it's because the
function defining the a0 is delayed...  is that right?

if so, how do i define a stepwise ode in Mathematica? I wanted to make a0= 0
at time 0, then 1/20 for given time then 0 and again opver next time
frame then 1/20 again for the rest of the time.

below is my attempt. but it brings back 

NDSolve::ndnum: Encountered non-numerical value for a derivative at t
== 0.


k1 = 1/10;
k2 = 1/20;
a0 := 0 /; t < 0 ;
a0 := 1/20 /; 0 <= t <= 200 ;
a0 := 0 /; 200 <= t <= 600 ;
a0 := 1/20 /; 600 <= t <= 1000; 

ndsolution =
NDSolve[{b'[t] == -k2 b[t] y[t], x'[t] == -k1 a0 x[t] + k2 b[t] y[t],
y'[t] == k1 a0 x[t] - k2 b[t] y[t], b[0] == 1, x[0] == 1, y[0] == 0},
{b, x, y}, {t, 0, 1000}]

thanks all very much in advance for any and all comments. 

sean


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