It looks to me as if it is likely that q and t represent vectors that are being indexed. You cannot use symsum() to index a vector. Instead use,
N = n+1;
j = (1:n) + 1;
XF = sum((q(j) - q(j-1)) / q(N) .* log(t(N) - t(j-1)));
The N = n+1 instead of n is present because the formula uses j = 1 : n and 
which would be 
when j = 1. MATLAB cannot index by 0, so we need to add 1 to each subscript.
which would be when j = 1. MATLAB cannot index by 0, so we need to add 1 to each subscript.Your tags mention integration, but this does not look like integration to me -- not unless it is Reimann integral of log(t_n - t) with independent variable stored in q ? Trapazoid calculations are generally more accurate than Reimann integral.
