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 A271956 #25 Jun 13 2017 10:36:44 %S A271956 1,4,6,10,35,112,149,390,424,566,660,664,850,1414,1436,1497,1524,1904, %T A271956 5116,6625,10944,12637,35691,92304 %N A271956 Numbers n such that 2*n*3^n - 1 is prime. %C A271956 No further terms < 10^5. %e A271956 1 is in this sequence because 2*1*3^1 - 1 = 5 is prime. %p A271956 A271956:=n->`if`(isprime(2*n*3^n-1), n, NULL): seq(A271956(n), n=1..2*10^3); # _Wesley Ivan Hurt_, Apr 17 2016 %t A271956 Select[Range[10000], PrimeQ[2 # 3^# - 1] &] %o A271956 (PARI) is(n)=ispseudoprime(2*n*3^n-1) \\ _Charles R Greathouse IV_, Jun 13 2017 %Y A271956 Cf. A006553, A266694. %K A271956 nonn,more %O A271956 1,2 %A A271956 _Ray Chandler_, Apr 17 2016