A064821 - OEIS

%I #15 Jun 05 2020 04:34:01

%S 0,1,1,4,2,8,12,30,70,664,1072,8768,11648,37108,95772,1059816,2047488,

%T 12111712,22802028,120779959,337801784,4361743860,11425028900,

%U 142573286216,429917269469,4138239848098,16171519847028,214845911686065

%N 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 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 I conjecture a(n) > 0 for all n.

%C Variant of A051239 with respect to a(1). - _R. J. Mathar_, Oct 02 2008

%e For n = 4 there are 4 sequences: 1234, 1432, 3214, 3412.

%Y Cf. A064821, A103839.

%K nonn,nice

%O 1,4

%A _N. J. A. Sloane_, Oct 23 2001

%E More terms from _Jud McCranie_, Oct 24 2001

%E a(22)-a(24) from _Donovan Johnson_, Aug 27 2010

%E a(25)-a(28) from _Giovanni Resta_, Jun 05 2020