Sigma functionThis page is about the sigma function that calculates the sum of the factors of a number. To see the page which discusses Rado's sigma function, see busy beaver.
The sigma function (not to be confused with summation) is the function that finds the sum of the positive divisors of a number. The sigma function is denoted by &sigma(n). If p is a prime number, σ(p) is p + 1 because, by definition, the factors of a prime number are 1 and itself.