proof of multiplication formula for gamma function
Define the function as
By the functional equation of the gamma function,
Hence is a periodic function of . However, for large valuesof , we can apply the Stirling approximation formula to conclude
Note that
Also,
Hence, . Now, the only way for a function to be periodic and have a definite limit is for that function to be constant. Therefore, . Writing out the definition of and rearranging gives the multiplication formula.