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 A104280 #18 Sep 08 2022 08:45:17 %S A104280 4,7,10,12,16,19,22,24,27,31,34,37,40,42,45,49,52,55,57,64,66,70,76, %T A104280 79,82,84,87,91,97,100,106,112,115,117,121,126,129,132,136,139,142, %U A104280 147,154,157,159,166,169,175,177,180,184,187,190,192,195,199,201,205,211,217 %N A104280 Numbers n such that 2n+1 is not prime and 2n-1 is prime. %t A104280 Select[ Range[219], !PrimeQ[2# + 1] && PrimeQ[2# - 1] &] (* _Robert G. Wilson v_, Apr 18 2005 *) %o A104280 (Magma) [n: n in [0..250]| not IsPrime(2*n+1) and IsPrime(2*n-1)] // _Vincenzo Librandi_, Jan 28 2011 %o A104280 (PARI) is(n)=isprime(2*n-1) && !isprime(2*n+1) \\ _Charles R Greathouse IV_, Jun 13 2017 %Y A104280 Cf. A040040 and others. %Y A104280 Equals A136798/2. %K A104280 easy,nonn %O A104280 1,1 %A A104280 _Alexandre Wajnberg_, Apr 17 2005 %E A104280 More terms from _Robert G. Wilson v_, Apr 18 2005