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 A080778 #46 Mar 19 2025 10:24:43 %S A080778 0,1,2,518,33416,37310,52608,123998,220502 %N A080778 Double factorial primes; values k for which k!! + 1 is prime. %C A080778 Numbers k such that A006882(k) + 1 is prime. %C A080778 a(1) and a(2) correspond to the same prime, 1+1. For n >= 3, a(n) is even (because (2j+1)!! + 1 is even), and then a(n)/2 equals A256594(n-1). - _Jeppe Stig Nielsen_, Apr 01 2023 %H A080778 Ken Davis, <a href="http://mfprimes.free-dc.org">Status of Search for Multifactorial Primes.</a> %H A080778 Ken Davis, <a href="http://mfprimes.free-dc.org/mfdata/f02p.html">Results for n!2+1.</a> %H A080778 Shyam Sunder Gupta, <a href="https://doi.org/10.1007/978-981-97-2465-9_16">Fascinating Factorials</a>, Exploring the Beauty of Fascinating Numbers, Springer (2025) Ch. 16, 411-442. %H A080778 Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/DoubleFactorialPrime.html">Double Factorial Prime</a> %H A080778 Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/IntegerSequencePrimes.html">Integer Sequence Primes</a> %e A080778 518 is included because 518*516*514*...*4*2 + 1 is prime. - _Jeppe Stig Nielsen_, Apr 01 2023 %Y A080778 Cf. A006882, A007749, A256594. %K A080778 hard,nonn,more %O A080778 1,3 %A A080778 _Steven Harvey_, Mar 11 2003 %E A080778 a(7) from Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 03 2008 %E A080778 a(8) from _Sou Fukui_, Jun 05 2015 %E A080778 a(9) from _Sou Fukui_, Jan 17 2017