The sigma symbol indicates the operation addition. It represents a summation notation where it is a shorthand method of writing an expression for a sum of values of a certain function. The subscript of the sigma symbol indicates the index of summation and the starting point for the operation or the lower limit of the summation. The superscript indicates the upper limit or the stopping point. So, for the expression
4∑(k=1) (-1) ^ k(k+11)
The lower limit is 1 and the upper limit is 4. It is the same as:
(-1)^1(1+11) + (-1)^2(2+11) + (-1)^3(3+11) + (-1)^4(4+11)
-1 + -1 + -1 + -1
-4