Reference: SIGMA-SUM

∑(expr, var, a, b) returns the sum expr(a) + expr(a + 1) + ... + expr(b) where expr is a string representing a real-valued expression in var. var is a string representing a valid identifier, used as the independent variable in expr. a and b are integers, corresponding to the first and last term of the summation, respectively; thus, there are exactly b − a + 1 terms in the sum.

Examples: ∑("1/n!", "n", 0, 100) = 2.71828182846