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