# Problem in trying to solve a differential equation [closed]

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?

• The error message is telling you didn't give any initial conditions in your NDSolve expression. – m_goldberg Jan 24 '16 at 9:03

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]

NDSolve[{u''[t] + u[t - 1] == 0,