sum of two gamma functions with the same shape parameter

ccl-ir

Hi
can any one state two gamma functions with the same shape parameter, in the form of a new gamma function or to somehow approximate that summation?

gamma(t,k) = (exp(-x/t)*x^(k-1))/(gamma(k)*(t^k))


a*gamma(t1,k)+b*gamma(t2,k) = ??? (??? in the form of a new gamma function)