The summation is performed over those integer values k for which the argument of each factorial in the denominator is non-negative, i.e. summation limits K and N are taken equal: the lower one K = max ( 0 , j 2 − − j 3 − − m 1 , j 1 − − j 3 + m 2 ) , the upper one N = min ( j 1 + j 2 − − j 3 , j 1 − − m 1 , j 2 + m 2 ) . Factorials of negative numbers are conventionally taken equal to zero, so that the values of the 3 j symbol at, for example, j 3 > j 1 + j 2 or j 1 < m 1 are automatically set to zero.