2δ(x − x0) is the Dirac delta “function”. It can be defined from the requirement that for every function f(x) we have that ∫ −+∞∞ f(x)δ(x − x0)dx = f(x0) . Obviously we also have that ∫+ ∞ δ(x − x0)dx = 1 −∞ . Intuitively one can think of it as a function that is almost zero everywhere except in an infinitesimal neighborhood of x0 .