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 A036456 Numbers k for which exactly 4 applications of A000005 are needed to reach 2. 2
 12, 18, 20, 24, 28, 30, 32, 40, 42, 44, 45, 48, 50, 52, 54, 56, 63, 66, 68, 70, 75, 76, 78, 80, 88, 92, 98, 99, 102, 104, 105, 110, 112, 114, 116, 117, 124, 128, 130, 135, 136, 138, 144, 147, 148, 152, 153, 154, 162, 164, 165, 170, 171, 172, 174, 175, 176, 182 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS Similar to but different from A007624. Terms like 60, 72, 84, 90, 96, 108, 126, etc. are not present here. LINKS Amiram Eldar, Table of n, a(n) for n = 1..10000 FORMULA With d(n) = number of divisors(n), d(d(d(d(a(n))))) = 2 and d(d(d(a(n)))) > 2. A036459(a(n)) = 4. - Ivan Neretin, Jan 25 2016 EXAMPLE a(3)=20 and a(17)=63; for both x=20 and 63, d(x)=6 and d(d(x))=4, the 3rd iterates are 3 and the equilibrium value, i.e., 2 appears as 4th iterates. PROG (PARI) isok(n) = ((nd=numdiv(n)) != 2) && ((nd=numdiv(nd)) != 2) && ((nd=numdiv(nd)) != 2) && ((nd=numdiv(nd)) == 2); \\ Michel Marcus, Dec 30 2013 & Jan 26 2015 CROSSREFS Cf. A000005, A007624, A036450, A036453, A036455, A046452. Sequence in context: A319229 A271345 A007624 * A102467 A126706 A123711 Adjacent sequences:  A036453 A036454 A036455 * A036457 A036458 A036459 KEYWORD nonn AUTHOR EXTENSIONS New name (using new name for A036457 from Robert Israel) from Jon E. Schoenfield, May 12 2018 STATUS approved

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Last modified April 3 20:29 EDT 2020. Contains 333199 sequences. (Running on oeis4.)