I have probably a silly question about correlation functions of composite operators. Why can't you just calculate a correlator with fields at different points x1, x2, x3, ... and then set a couple of the points equal at the end of the calculation to get the result? e.g., [tex]\langle 0 T\phi(x_1)\phi(x_2).... 0\rangle [/tex] and to have a phi^2 composite operators just set x1 equal to x2 at the end of the calculation? When you calculate [tex]\langle 0 T\phi(x)\phi(y).... 0\rangle [/tex] perturbatively at least, it seems the result is a fairly simple function of x and y. You'll get something like: constant*eikx*eiqy where k and q are integrated over. So just set x=y above?