[R] summation equation whose numerator has subscript

[Sherouk Moawad]

Dear R experts 
Please do you have any idea about how this summation can be written in R(the equation can be viewed in the following link): 
http://s16.postimg.org/or2km30ph/equation.jpg 

I've tried out out this code but it gave me error for writing brackets in function of summation:

>>> 
x=matrix(c(6,2,1),3,1) 

for (l in 1:3){ 
sum(sapply(1:3, function(j[l]){if(l>1){sum(sapply(1:j[l-1], function(j[l]){x[j[l]]*(j[l]<j[l-1])}))}}))}>>>Thank you