A065090 Natural numbers which are not odd primes: composites plus 1 and 2.

1, 2, 4, 6, 8, 9, 10, 12, 14, 15, 16, 18, 20, 21, 22, 24, 25, 26, 27, 28, 30, 32, 33, 34, 35, 36, 38, 39, 40, 42, 44, 45, 46, 48, 49, 50, 51, 52, 54, 55, 56, 57, 58, 60, 62, 63, 64, 65, 66, 68, 69, 70, 72, 74, 75, 76, 77, 78, 80, 81, 82, 84, 85, 86, 87, 88, 90, 91, 92, 93, 94

Offset: 1

Author

Labos Elemer, Nov 12 2001

Keywords

• nonn
• easy
• changed

Links

• Harry J. Smith, Table of n, a(n) for n = 1..1000

Crossrefs

Cf. A002808, A006005.

Complement of A065091.

Programs

Haskell

import Data.List (elemIndices)
a065090 n = a065090_list !! (n-1)
a065090_list = map (+ 1) $ elemIndices 0 a151763_list
-- Reinhard Zumkeller, Oct 06 2011

Mathematica

Join[{1, 2}, Select[Range[4, 100], CompositeQ]] (* Paolo Xausa, Aug 29 2025 *)

PARI

{ n=0; for (m=1, 10^9, if (!isprime(m) || m==2, write("b065090.txt", n++, " ", m); if (n==1000, return)) ) } \\ Harry J. Smith, Oct 06 2009

Python

from sympy import composite
def A065090(n): return composite(n-2) if n>2 else n # Chai Wah Wu, Jul 13 2024

Formula

A151763(a(n)) = 0.