[R] write a function inside the summation in a more condensed form

[S Ellison]

>  I want to write the inside function (3^(x[i]+x[j])) in a more condensed form
> cause this will help me when the multiple summations are more than two

indices<-c(i,j) #or whatever you want
3^sum( x[indices] )

S Ellison


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