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 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 (list; graph; refs; listen; history; text; internal format)
 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 Cf. A000203, A000040, A077374, A087998. Sequence in context: A272653 A030794 A134073 * A125237 A085459 A092169 Adjacent sequences:  A088002 A088003 A088004 * A088006 A088007 A088008 KEYWORD nonn AUTHOR Labos Elemer, Oct 18 2003 STATUS approved

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Last modified January 19 19:30 EST 2021. Contains 340270 sequences. (Running on oeis4.)