Composition of a theta function with function

[muppet]

Hi all,

I have some integrals involving Heaviside theta functions of non-trivial arguments, in particular $\theta(1-x^2)$ and $\theta(x^2-1)$. As x is a radial coordinate it's easy enough for me to break these up by hand, but it's cumbersome for me to implement in Mathematica and I can't work out how I'd go about attacking a more general problem. Is there an expression for $\theta(f(x))$ analagous to $$\delta(f(x))=\sum_i{\frac{\delta(x-x_i)}{|g'(x_i)|}}$$?
Thanks in advance.

 

[fresh_42]

Maybe one of the many formulas here can help: https://en.wikipedia.org/wiki/Theta_function