Sigma function

In mathematics, by sigma function one can mean one of the following:

• The sum-of-divisors function σ_a(n), an arithmetic function

And the formula to find its value is given as follow: Say n=∏[p(i)]^[e(i)] where i=1 to i=k Then σ(n)=∏{([p(i)]^[e(i)+1])/[p(i)-1]} where this product is from i=1 to i=k

• Weierstrass sigma function, related to elliptic functions
• Rado's sigma function, see busy beaver

See also sigmoid function.