[R] Tail area of sum of Chi-square variables

[Klaus Nordhausen]

Dear R experts, 

I was wondering if there are any R functions that give the tail area
of a sum of chisquare distributions of the type:
         a_1 X_1 + a_2 X_2
where a_1 and a_2 are constants and X_1 and X_2 are independent chi-square variables with different degrees of freedom.

Thanks,

Klaus
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