One of the first two terms on the right should have an i+1 rather than i-1, but this is undoubtedly not the problem that caused the error message. The code will probably work for most values of i and j, but it has problems at the borders of the array T1(). For example, for i=1, the first term evaluates to T1(0,j), and you can't have a subscript of zero. Likewise, at the maximum value imax of i, the subscripts should evaluation to T1(imax+1,j), and this will be out of bounds for the array. The same applies to the bordering values for j. So, in this type of expression, you have to treat the edges of the arrays separately.