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 A084307 a(n) is the least number x such that gcd(sigma(x), sigma(x+1)) = n. 2
 1, 13, 17, 6, 199, 5, 242, 27, 391, 57, 1296, 22, 882, 12, 648, 93, 175232, 89, 3872, 236, 195, 1032, 4875263, 14, 5739271, 467, 35377, 83, 1882384, 58, 3024, 308, 29240, 201, 1627208, 118, 79524, 147, 1682, 56, 46854024, 82, 229441, 1204, 2047, 6301, 83386957823 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS a(47) > 10^9 if it exists. - Michel Marcus, Sep 01 2019 LINKS EXAMPLE n=9: GCD[sigma[x+1], sigma[x]]=5 holds for {391,799,800,881,...} of which the first is a(9)=391. MATHEMATICA f[x_] := GCD[DivisorSigma[1, x], DivisorSigma[1, x+1]] t=Table[0, {256}]; Do[s=f[n]; If[s<257&&t[[s]]==0, t[[s]]=n], {n, 1, 10000000}]; t PROG (PARI) a(n) = my(x=1, sx=sigma(x), y=2, sy=sigma(2)); while(gcd(sx, sy) != n, x++; y++; sx=sy; sy=sigma(y)); x; \\ Michel Marcus, Aug 28 2019 CROSSREFS Cf. A000203, A060780, A063444. Sequence in context: A219949 A173632 A061060 * A066918 A164062 A338371 Adjacent sequences: A084304 A084305 A084306 * A084308 A084309 A084310 KEYWORD nonn AUTHOR Labos Elemer, Jun 13 2003 EXTENSIONS a(41) from Rémy Sigrist, Aug 29 2019 a(42)-a(46) from Michel Marcus, Aug 30 2019 a(47) from Giovanni Resta, Oct 29 2019 STATUS approved

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Last modified February 3 07:33 EST 2023. Contains 360024 sequences. (Running on oeis4.)