[pstricks] plot of the sum of the second derivative of a gamma function

[Cyrille Piatecki]

Dear all,

I ask for a little help  I need to plot the second derivative of the  
sum of two gamma   distribution functions ---  
$F_1^{\prime\prime}+F_2^{\prime\prime}$   such that :

\[ F^\prime_1 = \Gamma(x)^{-1}\left[\lambda_1 e^{- \lambda_1 x}    
(\lambda_1 x)^(t_1-1)\right]
\]

and
\[ F^\prime_2 = \Gamma(x)^{-1}\left[\lambda_2 e^{- \lambda_2 x}    
(\lambda_1 (x-\mu))^(t_2-1)\right]
\]

where incidentaly $\mu$ is the mean of $F_1$ but this is not a problem  
   since it is easy to know the expectation from a Gamma distribution   
  from its parameters.

Thanks for the help  Cyrille Piatecki