A036308 Composite numbers whose prime factors contain no digits other than 1 and 8.

121, 1331, 1991, 8921, 9691, 12991, 14641, 19921, 21901, 32761, 89221, 98131, 106601, 142901, 146791, 159461, 161051, 199991, 213761, 219131, 240911, 327791, 360371, 657721, 714491, 776161, 892991, 957791, 976921, 981431, 1040461, 1079441, 1172611, 1394761, 1468091

Offset

1,1

Comments

All terms are a product of at least two terms of A020456. - David A. Corneth, Oct 09 2020

Links

David A. Corneth, Table of n, a(n) for n = 1..10000 (first 58 terms from Vincenzo Librandi)

Index entries for sequences related to prime factors.

Formula

Sum_{n>=1} 1/a(n) = Product_{p in A020456} (p/(p - 1)) - Sum_{p in A020456} 1/p - 1 = 0.0101097220... . - Amiram Eldar, May 18 2022

Mathematica

dpfQ[n_]:=Module[{d=Union[Flatten[IntegerDigits/@Transpose[FactorInteger[n]][[1]]]]}, !PrimeQ[n]&&(d == {1}||d == {8}||d == {1, 8})]; Select[Range[2, 1500000], dpfQ] (* Vincenzo Librandi, Aug 25 2013 *)

Prog

(Magma) [n: n in [4..1500000] | not IsPrime(n) and forall{f: f in PrimeDivisors(n) | Intseq(f) subset [1, 8]}]; // Bruno Berselli, Aug 26 2013

Crossrefs

Cf. A020456, A036302-A036325.

Sequence in context: A088171 A357169 A254970 * A066444 A206466 A137434

Adjacent sequences: A036305 A036306 A036307 * A036309 A036310 A036311

Keyword

nonn,easy,base

Author

Patrick De Geest, Dec 15 1998

Extensions

More terms from Vincenzo Librandi, Aug 25 2013

Status

approved