OFFSET
1,1
COMMENTS
Closed under multiplication. - David A. Corneth, Sep 21 2020
From M. F. Hasler, Sep 22 2020: (Start)
Also closed under LCM, but not under GCD.
All terms are congruent to 1 or 9 (mod 10), depending on the parity of their number of prime factors counted with multiplicity, A001222. (End)
LINKS
FORMULA
Sum_{n>=1} 1/a(n) = Product_{p in A020466} (p/(p - 1)) - Sum_{p in A020466} 1/p - 1 = 0.00001523788893... . - Amiram Eldar, May 22 2022
EXAMPLE
The smallest prime made up of 4's and 9's is 449 (see A020466), so the smallest term here is 449^2 = 201601. - N. J. A. Sloane, Sep 21 2020
MATHEMATICA
cn49Q[n_]:=Module[{fi=FactorInteger[n][[All, 1]]}, CompositeQ[n]&&Union[ Flatten[ IntegerDigits/@fi]]=={4, 9}&&AllTrue[fi, PrimeQ]]; Select[Range[ 1, 1006*10^5, 2], cn49Q] (* Requires Mathematica version 10 or later *) (* Harvey P. Dale, Sep 21 2020 *)
PROG
(PARI) is(N)={!isprime(N)&& !#setminus(Set(concat(apply (digits, factor(N)[, 1]))), [4, 9])} \\ M. F. Hasler, Sep 22 2020
CROSSREFS
KEYWORD
nonn,easy,base
AUTHOR
Patrick De Geest, Dec 15 1998
EXTENSIONS
More terms from David A. Corneth, Sep 21 2020
STATUS
approved