A080689 Powers of 10 that reach ...,7,8,4,2,1 under the mapping: if n is even divide by 2 else add 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

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