This means that our integral becomes I=∫∞0r21dr1∫∞0r22dr2∫π0dcos(θ1)∫π0dcos(θ2)∫2π0dϕ1∫2π0dϕ2exp−2α(r1+r2)r12 where we have defined 1r12=1√r21+r22−2r1r2cos(β) with cos(β)=cos(θ1)cos(θ2)+sin(θ1)sin(θ2)cos(ϕ1−ϕ2))