Reference: PI-PRODUCT

∏(expr, var, a, b) returns the product 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 factor of the product, respectively; thus, there are exactly b − a + 1 factors in the product.