a_f = INTEGRAL a_g ( INTEGRAL F_f F_g ) dq dg

a is the coefficient, F = F(q) is an eigenfunction.

From this it is shown that the first integral (ie. of the eigenfunctions with dq) must be a Dirac delta function, that is, that for f = g it is infinite. Why is this? Landau Lifgarbagez states that it is to prevent the integral with dg from vanishing, but I don't see this.

This would mean that a_f = INTEGRAL a_f (infinity) df ....how is this?

Cheer folks!!! :)