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 A051239 #16 Jul 03 2017 02:13:43 %S A051239 1,1,1,4,2,8,12,30,70,664,1072,8768,11648,37108,95772,1059816,2047488, %T A051239 12111712,22802028,120779959,337801784,4361743860,11425028900, %U A051239 142573286216,429917269469,4138239848098,16171519847028,214845911686065 %N A051239 Number of ways to arrange integers 1 through n so that the sum of each adjacent pair is prime, not counting reversals. %C A051239 Similar to A036440, but not requiring 1 to be first and n to be last. %F A051239 For n>1, a(n) = A103839(n)/2. %e A051239 a(5)=2 since 1,4,3,2,5 and 3,4,1,2,5 have the property that the sum of each adjacent pair is prime. %Y A051239 Cf. A036440, A051237. %K A051239 nonn,nice %O A051239 1,4 %A A051239 _Jud McCranie_ %E A051239 More terms from _Max Alekseyev_, Jan 04 2008 %E A051239 a(25)-a(28) from _Giovanni Resta_, Apr 01 2014