A064821 - OEIS

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.

0, 1, 1, 4, 2, 8, 12, 30, 70, 664, 1072, 8768, 11648, 37108, 95772, 1059816, 2047488, 12111712, 22802028, 120779959, 337801784, 4361743860, 11425028900, 142573286216, 429917269469, 4138239848098, 16171519847028, 214845911686065

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OFFSET

1,4

COMMENTS

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.

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

Variant of A051239 with respect to a(1). - R. J. Mathar, Oct 02 2008

LINKS

Table of n, a(n) for n=1..28.

EXAMPLE

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

CROSSREFS