A053181 Composite numbers ending in 9.

9, 39, 49, 69, 99, 119, 129, 159, 169, 189, 209, 219, 249, 259, 279, 289, 299, 309, 319, 329, 339, 369, 399, 429, 459, 469, 489, 519, 529, 539, 549, 559, 579, 589, 609, 629, 639, 649, 669, 679, 689, 699, 729, 749, 759, 779, 789, 799, 819, 849, 869, 879, 889, 899

Offset: 1

Author

Enoch Haga, Feb 29 2000

Keywords

• easy
• nonn
• base

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

Programs

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 *)

PARI

forstep(n=9,1e3,10,if(!isprime(n),print1(n", "))) \\ Charles R Greathouse IV, Jun 24 2011

Formula

a(n) ~ 10n. - Charles R Greathouse IV, Jun 24 2011

Extensions

9 added by Kausthub Gudipati, Jun 16 2011