A139270 Twice nonprime numbers.

2, 8, 12, 16, 18, 20, 24, 28, 30, 32, 36, 40, 42, 44, 48, 50, 52, 54, 56, 60, 64, 66, 68, 70, 72, 76, 78, 80, 84, 88, 90, 92, 96, 98, 100, 102, 104, 108, 110, 112, 114, 116, 120, 124, 126, 128, 130, 132, 136, 138, 140, 144, 148, 150

Offset: 1

Author

Omar E. Pol, May 16 2008

Keywords

• easy
• nonn

Comments

Besides a(1), is this sequence a subset of A091375? - Bill McEachen, Dec 18 2016

Crossrefs

Cf. A018252.

Programs

Mathematica

2*Select[Range[75],!PrimeQ[#]&] (* Stefano Spezia, Nov 18 2023 *)

PARI

is(n)=n%2==0 && !isprime(n/2) && n \\ Charles R Greathouse IV, Feb 21 2017

Python

from sympy import composite
def A139270(n): return composite(n-1)<<1 if n>1 else 2 # Chai Wah Wu, Oct 15 2024

Formula

a(n) = A018252(n)*2.