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 A113285 Let S(n)=sigma(|n|)-2*n; sequence gives numbers n such that S(S(S(S(n))))=n. May be called {2,1}-Sociable number of orders 1 or 2 or 4. 3
 51, 72, 120, 132, 672, 2602, 4756, 10054, 14884, 45840, 51168, 116252, 523776, 906202, 3003698, 5271836, 65071776, 77260656, 82842816, 89761152, 138357404, 139626548, 459818240, 985948800, 1381340160, 1476304896, 1489384608, 2183133696, 3835877062 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS Orders of cycles are 4,4,1,4,1,4,4,4,4,2,2,2,1,2,4,4,4,4,4,4,4,4,... LINKS Eric Weisstein's World of Mathematics, Sociable Numbers EXAMPLE {51,-30,132,72} is a {2,1}-Aliquot cycle. MATHEMATICA fQ[n_] := Nest[ DivisorSigma[1, # ] - 2# &, n, 4] == n; t = {}; Do[ If[ fQ[n], AppendTo[t, n]], {n, 3*10^7}]; t (* Robert G. Wilson v *) CROSSREFS Cf. A113791, A114528, A114529. Sequence in context: A232105 A225801 A242045 * A050698 A039474 A253020 Adjacent sequences:  A113282 A113283 A113284 * A113286 A113287 A113288 KEYWORD nonn AUTHOR Yasutoshi Kohmoto, Jan 27 2006 EXTENSIONS a(12)-a(22) from Robert G. Wilson v, Jan 30 2006 a(23)-a(29) from Charles R Greathouse IV, Nov 13 2010 STATUS approved

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Last modified May 7 02:20 EDT 2021. Contains 343636 sequences. (Running on oeis4.)