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 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 (list; graph; refs; listen; history; text; internal format)
 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 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

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Last modified August 2 18:52 EDT 2021. Contains 346428 sequences. (Running on oeis4.)