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A016028
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Expansion of (1 - x + x^4) / (1 - x)^3.
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6
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1, 2, 3, 4, 6, 9, 13, 18, 24, 31, 39, 48, 58, 69, 81, 94, 108, 123, 139, 156, 174, 193, 213, 234, 256, 279, 303, 328, 354, 381, 409, 438, 468, 499, 531, 564, 598, 633, 669, 706, 744, 783, 823, 864, 906, 949, 993, 1038, 1084, 1131, 1179
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OFFSET
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1,2
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COMMENTS
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For n>2, maximal number of edges in critical strongly connected digraphs on n-1 vertices.
If Y is a 3-subset of an n-set X then, for n>=3, a(n) is the number of 2-subsets of X which do not have exactly one element in common with Y. Also, if Y is a 3-subset of an n-set X then, for n>=4, a(n-3) is the number of (n-2)-subsets of X which have no exactly two elements in common with Y. - Milan Janjic, Dec 28 2007
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LINKS
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FORMULA
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a(n) = binomial(n,2) - 3*n + 9, n>=3. a(n-3) = n^2/2 - 7*n/2 + 9, n>=4. - Milan Janjic, Dec 28 2007
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MATHEMATICA
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CoefficientList[Series[(1-x+x^4)/(1-x)^3, {x, 0, 50}], x] (* or *) LinearRecurrence[{3, -3, 1}, {1, 2, 3, 4, 6}, 60] (* Harvey P. Dale, Nov 30 2015 *)
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PROG
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CROSSREFS
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KEYWORD
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nonn,easy
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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