login
A088005
Numbers whose abundance is (+-1) times a prime.
6
3, 9, 10, 18, 20, 25, 36, 49, 50, 81, 100, 104, 121, 136, 196, 225, 289, 324, 338, 392, 464, 484, 576, 650, 676, 784, 800, 841, 900, 961, 968, 1058, 1089, 1156, 1225, 1250, 1352, 1682, 1936, 1952, 2209, 2312, 2450, 2500, 2888, 3136, 3364, 3721, 4225, 4356
OFFSET
1,1
LINKS
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