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I'm trying to solve a differential equation but not getting a solution.

Some references I read gave me hits about using a runge kutta method. I'm new to the software, any hints regarding this error or how to solve?

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  • $\begingroup$ The error message is telling you didn't give any initial conditions in your NDSolve expression. $\endgroup$ – m_goldberg Jan 24 '16 at 9:03
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The $\theta(t-\Delta)$ makes it a Delay Differential Equation, which the documentation here shows how to solve. You have to specify a history function like this:

u[t /; t <= 0] == f[t]

For example:

NDSolve[{u''[t] + u[t - 1] == 0, 
    u[t /; t <= 0] == 1},
    u, {t, 0, 10}]
Plot[u[t] /. First[%], {t, 0, 10}]
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