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 A064821 #15 Jun 05 2020 04:34:01
%S A064821 0,1,1,4,2,8,12,30,70,664,1072,8768,11648,37108,95772,1059816,2047488,
%T A064821 12111712,22802028,120779959,337801784,4361743860,11425028900,
%U A064821 142573286216,429917269469,4138239848098,16171519847028,214845911686065
%N A064821 Number of ways of writing the numbers 1 .. n in a sequence so that the sum of any two adjacent numbers is a prime; reversing the sequence does not count as different.
%C A064821 If the sequence is d_1 d_2 ... d_n then the n-1 sums d_i + d_{i+1} are required to be primes.
%C A064821 I conjecture a(n) > 0 for all n.
%C A064821 Variant of A051239 with respect to a(1). - _R. J. Mathar_, Oct 02 2008
%e A064821 For n = 4 there are 4 sequences: 1234, 1432, 3214, 3412.
%Y A064821 Cf. A064821, A103839.
%K A064821 nonn,nice
%O A064821 1,4
%A A064821 _N. J. A. Sloane_, Oct 23 2001
%E A064821 More terms from _Jud McCranie_, Oct 24 2001
%E A064821 a(22)-a(24) from _Donovan Johnson_, Aug 27 2010
%E A064821 a(25)-a(28) from _Giovanni Resta_, Jun 05 2020