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A036310
Composite numbers whose prime factors contain no digits other than 2 and 3.
2
4, 6, 8, 9, 12, 16, 18, 24, 27, 32, 36, 46, 48, 54, 64, 69, 72, 81, 92, 96, 108, 128, 138, 144, 162, 184, 192, 207, 216, 243, 256, 276, 288, 324, 368, 384, 414, 432, 446, 466, 486, 512, 529, 552, 576, 621, 648, 669, 699, 729, 736, 768, 828, 864, 892, 932, 972
OFFSET
1,1
COMMENTS
All terms are a product of at least two terms of A020458. - David A. Corneth, Oct 09 2020
LINKS
David A. Corneth, Table of n, a(n) for n = 1..10000 (first 1000 terms from Vincenzo Librandi)
FORMULA
Sum_{n>=1} 1/a(n) = Product_{p in A020458} (p/(p - 1)) - Sum_{p in A020458} 1/p - 1 = 1.2804224166... . - Amiram Eldar, May 18 2022
MATHEMATICA
dpfQ[n_]:=Module[{d=Union[Flatten[IntegerDigits/@Transpose[ FactorInteger[ n]][[1]]]]}, !PrimeQ[n] && (d=={2}||d=={3}||d=={2, 3})]; Select[Range[ 1000], dpfQ] (* Harvey P. Dale, Aug 24 2013 *)
CROSSREFS
KEYWORD
nonn,easy,base
AUTHOR
Patrick De Geest, Dec 15 1998
STATUS
approved