R help - Apr 2016 - write a function inside the summation in a more condensed form

Dear R Experts
that's my original equation
> x=c(2,4)
>
Y1=sum(sapply(1:2,function(i){sum(sapply(1:i,function(j){(3^(x[i]+x[j]))}))}))
> y1 
[1] 7371 
 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

I've tried the following form
>
y2=sum(sapply(1:2,function(i){sum(sapply(1:i,function(j){(3^(sum(sapply(i:j,function(l){x[l]}))))}))}))
> y2 
[1] 819 


But it didn't work, it gave another solution 
Any help or recommendations

>  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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