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Search: id:A064821
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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
1
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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
(list; graph; listen)
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OFFSET
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1,4
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COMMENT
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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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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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Adjacent sequences: A064818 A064819 A064820 this_sequence A064822 A064823 A064824
Sequence in context: A109816 A050128 A134042 this_sequence A051239 A002291 A110622
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KEYWORD
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nonn,nice
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AUTHOR
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njas, Oct 23 2001
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EXTENSIONS
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More terms from Jud McCranie (j.mccranie(AT)comcast.net), Oct 24 2001
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