[math-fun] product_{(k,n)=1} Γ(z+k/n) ?

Someone once sent me (us?) a paper deriving (presumably by inclusion-exclusion) this gamma product formula. Does anyone remember where it is? Presumably for n=12 it gives something like (z - 11/12)!*(z - 7/12)!*(z - 5/12)!*(z - 1/12)!== 4*(2*z)!*(12*z)!*π^2/(2^(12*z)*3^(6*z)*(4*z)!*(6*z)!) It doesn't seem to be on http://functions.wolfram.com/ --rwg