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A064821
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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.
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0
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0, 1, 1, 4, 2, 8, 12, 30, 70, 664, 1072, 8768, 11648, 37108, 95772, 1059816, 2047488, 12111712, 22802028, 120779959, 337801784, 4361743860, 11425028900, 142573286216
(list; graph; refs; listen; history; internal format)
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OFFSET
| 1,4
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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). [From R. J. Mathar (mathar(AT)strw.leidenuniv.nl), Oct 02 2008]
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EXAMPLE
| For n = 4 there are 4 sequences: 1234, 1432, 3214, 3412.
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CROSSREFS
| Sequence in context: A050128 A134042 A051239 * A002291 A110622 A130078
Adjacent sequences: A064818 A064819 A064820 * A064822 A064823 A064824
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KEYWORD
| nonn,nice
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AUTHOR
| N. J. A. Sloane (njas(AT)research.att.com), Oct 23 2001
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EXTENSIONS
| More terms from Jud McCranie (JudMcCranie(AT)ugaalum.uga.edu), Oct 24 2001
a(22)-a(24) from Donovan Johnson (donovan.johnson(AT)yahoo.com), Aug 27 2010
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