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 A080689 Powers of 10 that reach ...,7,8,4,2,1 under the mapping: if n is even divide by 2 else add 1. 1
 2, 5, 18, 21, 24, 27, 30, 33, 36, 49, 52, 55, 58, 61, 64, 77, 80, 83, 86, 89, 92, 105, 108, 111, 114, 117, 120, 123, 136, 139, 142, 145, 148, 151, 164, 167, 170, 173, 176, 179, 182, 195, 198, 201, 204, 207, 210, 223, 226, 229, 232, 235, 238, 251, 254, 257, 260 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 LINKS G. C. Greubel, Table of n, a(n) for n = 1..1000 MATHEMATICA a[n_] := MemberQ[Partition[NestWhileList[If[EvenQ[#], #/2, # + 1] &, 10^n, # != 1 &], 5, 1], {7, 8, 4, 2, 1}]; Select[Range[500], a] (* G. C. Greubel, Apr 12 2017 *) PROG (PARI) xnp10(n, p) = { for(x=1, n, p10 = 10^x; while(p10>1, if(p10%2==0, p10/=2, p10 = p10*p+1; ); if(p10==7, print1(x" ")) ) ) } CROSSREFS Cf. A022803. Sequence in context: A258429 A117839 A269665 * A026321 A288994 A226141 Adjacent sequences:  A080686 A080687 A080688 * A080690 A080691 A080692 KEYWORD easy,nonn AUTHOR Cino Hilliard, Mar 23 2003 STATUS approved

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Last modified May 28 17:37 EDT 2020. Contains 334684 sequences. (Running on oeis4.)