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A122977
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Number of sublattices of the divisor lattice of divisors of n that include n.
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5
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1, 2, 2, 4, 2, 7, 2, 8, 4, 7, 2, 21, 2, 7, 7, 16, 2, 21, 2, 21, 7, 7, 2, 58, 4, 7, 8, 21, 2, 45, 2, 32, 7, 7, 7, 84, 2, 7, 7, 58, 2, 45, 2, 21, 21, 7, 2, 152, 4, 21, 7, 21, 2, 58, 7, 58, 7, 7, 2, 200, 2, 7, 21, 64, 7, 45, 2, 21, 7, 45, 2, 293, 2, 7, 21, 21, 7, 45, 2, 152, 16, 7, 2, 200, 7, 7
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OFFSET
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1,2
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COMMENTS
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A divisor lattice is closed under GCD and LCM. First differences of A074986. Depends only on the prime signature of n.
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LINKS
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FORMULA
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EXAMPLE
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The a(6) = 7 sublattices of {1,2,3,6} that include 6 are: {6}, {1,6}, {2,6}, {3,6}, {1,2,6}, {1,3,6}, {1,2,3,6}.
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MATHEMATICA
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okQ[dd_List] := AllTrue[Subsets[dd, {2}], MemberQ[dd, GCD @@ #] && MemberQ[dd, LCM @@ #]&];
a[n_] := Select[Rest @ Subsets[Divisors[n]], Last[#] == n && okQ[#]&] // Length;
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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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STATUS
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approved
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