Can somebody help me out? I'm reading about formulas for cross sections for spin1 particles but I don't understand the delta functions, in calculating the 2particle pahse space psi For example the interaction; A+B -> C+D has the formula; psi= (2pi)^2 delta(Pa+Pb-Pc-Pd) d3Pc d3Pd / 4EcEd then all the books say the same; delta(Pa+Pb-Pc-Pd) d3Pd = delta (Ea+Eb-Ec-Ed) can somebody explain this to me? the left side delta is zero everywhere pa+pb =not pc+pd. the integral over the delta function d3Pd is; 1 for pd<pa+pb-pc and 0 for pd>pa+pb-pc ??? how do I use this?