OFFSET
1,1
LINKS
Amiram Eldar, Table of n, a(n) for n = 1..10000
FORMULA
abs(sigma(n)-2n) is a prime.
EXAMPLE
n=18: sigma[18]=18+9+6+3+2+1=39, 2n=36, abundance=3, a prime;
n=25: sigma[25]=25+5+1=31, 2n=50, abundance=-19.
MATHEMATICA
ab[x_] := DivisorSigma[1, x]-2*x Do[If[PrimeQ[s=ab[n]], Print[{n, s}]], {n, 1, 10000}]
PROG
(PARI) isok(n) = isprime(abs(sigma(n)-2*n)); \\ Michel Marcus, Sep 21 2019
CROSSREFS
KEYWORD
nonn
AUTHOR
Labos Elemer, Oct 18 2003
STATUS
approved