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 A067812 #8 Jul 02 2025 16:02:01 %S A067812 1,6,8,9,14,15,18,20,21,26,30,33,35,36,39,44,48,50,51,54,56,63,65,68, %T A067812 69,74,75,78,81,86,90,95,96,98,99,105,111,114,116,119,120,125,128,134, %U A067812 135,138,140,141,146,153,155,156,158,165,168,174,176,183,186,189,194 %N A067812 Nonprime n such that 2n+1 is prime. %e A067812 21*2+1 = 43, a prime, so 21 belongs to the sequence. %p A067812 with(numtheory): for n from 2 to 500 do if tau(n) > 2 and isprime(2*n+1) then printf(`%d,`,n) fi: od: %t A067812 Select[Range[200], PrimeQ[2#+1]&&!PrimeQ[ # ]&] %Y A067812 Cf. A005384. %K A067812 easy,nonn %O A067812 1,2 %A A067812 _Joseph L. Pe_, Feb 07 2002 %E A067812 Edited by _Dean Hickerson_ and _James Sellers_, Feb 12 2002