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A096158
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Number of permutations of proper divisors of n such that the sum of adjacent pairs of divisors is prime.
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2
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0, 0, 0, 2, 0, 2, 0, 2, 0, 2, 0, 4, 0, 0, 0, 0, 0, 0, 0, 0, 0, 2, 0, 0, 0, 0, 0, 0, 0, 4, 0, 0, 0, 2, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 2, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 2, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0
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OFFSET
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1,4
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COMMENTS
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For n>4: a(n)=2 iff (n/2,2+n/2) is twin prime pair, a(2*A001359(n))=2;
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LINKS
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EXAMPLE
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Proper divisors of n=12 are {1,2,3,4,6}:
[2,3,4,1,6]->(2+3,3+4,4+1,1+6)=(5,7,5,7),
[4,3,2,1,6]->(4+3,3+2,2+1,1+6)=(7,5,3,7),
[6,1,2,3,4]->(6+1,1+2,2+3,3+4)=(7,3,5,7) and
[6,1,4,3,2]->(6+1,1+4,4+3,3+2)=(7,5,7,5): therefore a(12)=4.
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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STATUS
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approved
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