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A Greek sigma with a subscript, summing powers of divisors.
Also known as: Sigma function, Sum of divisors function, Divisor sigma
σ_k(n) adds up the k-th powers of the divisors of n. With k zero it counts them; with k one it sums them, and σ(n) = 2n is exactly the definition of a perfect number — which is where the notation touches something a reader already knows.
Denotes the sum of the k-th powers of the divisors of a number, the subscript naming the power.
Contexts: Mathematics
With subscript zero counts divisors and with subscript one sums them; a number equal to twice its own divisor sum is perfect.
Contexts: Mathematics
A Greek lowercase sigma — a small loop with a horizontal stroke leading right from its top — with a small numeral below and to the right, followed by a bracketed numeral.