j_dim_sum
takes a three dimensional integer vector, fixes the values
of the first and third dimensions, and then sums the values of all the
entries in the vector with those two values for its first and third
dimensions.
1  j_dim_sum(n, i, k)

n 
A three dimensional integer vector. 
i 
An integer that is a valid index of the first dimension of

k 
An integer that is a valid index of the third dimension of

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