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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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3
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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, 429917269469, 4138239848098, 16171519847028, 214845911686065
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OFFSET
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1,4
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COMMENTS
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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.
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LINKS
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EXAMPLE
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For n = 4 there are 4 sequences: 1234, 1432, 3214, 3412.
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CROSSREFS
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KEYWORD
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nonn,nice
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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