Question: what does " i1~=1 " mean ?

after differentiating a function that contains a product and summation using a counter i=1,2,...,n and j=1,2,...m    i got a result that contains things like these :-

sum from i1~=1  to  i-1    and  i1~=i+1  to  n

and i had in the function a variable z[i][j]  and in the result it became  z[i1~][j]

what does 1~ means ?

thankx in advance

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