This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A139270 #21 Oct 15 2024 15:43:14 %S A139270 2,8,12,16,18,20,24,28,30,32,36,40,42,44,48,50,52,54,56,60,64,66,68, %T A139270 70,72,76,78,80,84,88,90,92,96,98,100,102,104,108,110,112,114,116,120, %U A139270 124,126,128,130,132,136,138,140,144,148,150 %N A139270 Twice nonprime numbers. %C A139270 Besides a(1), is this sequence a subset of A091375? - _Bill McEachen_, Dec 18 2016 %F A139270 a(n) = A018252(n)*2. %t A139270 2*Select[Range[75],!PrimeQ[#]&] (* _Stefano Spezia_, Nov 18 2023 *) %o A139270 (PARI) is(n)=n%2==0 && !isprime(n/2) && n \\ _Charles R Greathouse IV_, Feb 21 2017 %o A139270 (Python) %o A139270 from sympy import composite %o A139270 def A139270(n): return composite(n-1)<<1 if n>1 else 2 # _Chai Wah Wu_, Oct 15 2024 %Y A139270 Cf. A018252. %K A139270 easy,nonn %O A139270 1,1 %A A139270 _Omar E. Pol_, May 16 2008