I have a question about equation 10.21 in "Exact Space-times in Einstein's General Relativity" by Griffiths and Podolski. The equation is the well know standard metric for multiple extreme Reissner Nordstrom black holes. It has the below term: (1+ sigma(mi/ri)) The point ri = 0 is of course the surface of the hole. My questions is the book states "The analytically extended region inside this horizon has a metric similar to "10.21" but with a negative sign in the corresponding term in the sum, ... " They don't give a reference. Does anyone know how to I derive the above result? Thanks.