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A036308 Composite numbers whose prime factors contain no digits other than 1 and 8. 2
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 (list; graph; refs; listen; history; text; internal format)
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: A036928 A088171 A254970 * A176770 A066444 A206466

Adjacent sequences:  A036305 A036306 A036307 * A036309 A036310 A036311

KEYWORD

nonn,easy,base,changed

AUTHOR

Patrick De Geest, Dec 15 1998

EXTENSIONS

More terms from Vincenzo Librandi, Aug 25 2013

STATUS

approved

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Last modified May 22 16:57 EDT 2022. Contains 353957 sequences. (Running on oeis4.)