

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.


3



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 n1 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

Cf. A064821, A103839.
Sequence in context: A246202 A134042 A051239 * A002291 A225872 A231777
Adjacent sequences: A064818 A064819 A064820 * A064822 A064823 A064824


KEYWORD

nonn,nice


AUTHOR

N. J. A. Sloane, Oct 23 2001


EXTENSIONS

More terms from Jud McCranie, Oct 24 2001
a(22)a(24) from Donovan Johnson, Aug 27 2010
a(25)a(28) from Giovanni Resta, Jun 05 2020


STATUS

approved



