OFFSET
1,1
COMMENTS
Numbers of the form 10n + 9 (A017377) not in A030433. Obviously multiples of 2 or 5 can't be in A017377 nor this sequence. All other primes occur as factors of these terms infinitely often. For example: If n is a multiple of 3, then 10n + 9 is in this sequence; if n = 4 mod 7, then 10n + 9 is in this sequence; if n = 9 mod 11, then 10n + 9 is in this sequence; and so on and so forth. The first of these congruences to be satisfied determines the least prime factor of 10n + 9. - Alonso del Arte, Jun 23 2011
LINKS
G. C. Greubel, Table of n, a(n) for n = 1..10000
FORMULA
a(n) ~ 10n. - Charles R Greathouse IV, Jun 24 2011
MATHEMATICA
Complement[Range[9, 1000, 10], Prime[Range[PrimePi[1000]]]] (* Alonso del Arte, Jun 23 2011 *)
Select[Range[1000], CompositeQ[#]&&Mod[#, 10]==9&] (* Harvey P. Dale, May 01 2016 *)
PROG
(PARI) forstep(n=9, 1e3, 10, if(!isprime(n), print1(n", "))) \\ Charles R Greathouse IV, Jun 24 2011
CROSSREFS
KEYWORD
easy,nonn,base
AUTHOR
Enoch Haga, Feb 29 2000
EXTENSIONS
9 added by Kausthub Gudipati, Jun 16 2011
STATUS
approved