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Search: id:A016028
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| A016028 |
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Expansion of (1 - x + x^4) / (1 - x)^5. |
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+0
3
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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
(list; graph; listen)
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OFFSET
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1,2
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COMMENT
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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 have no 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 R. Janjic (agnus(AT)blic.net), Dec 28 2007
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LINKS
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R. Aharoni and E. Berger, [math/9911113] The number of edges in critical strongly connected graphs
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FORMULA
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Also, from the third term on, triangular numbers + 3 - Alexandre Wajnberg (alexandre.wajnberg(AT)skynet.be), Dec 10 2005
a(n)=binomial(n,2)-3*n+9, n=3,4,5,.... a(n-3)=n^2/2-7*n/2+9, n=4,5,6,.... - Milan R. Janjic (agnus(AT)blic.net), Dec 28 2007
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MATHEMATICA
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i=0; s=3; lst={1, 2}; Do[s+=n+i; AppendTo[lst, s], {n, 0, 6!, 1}]; lst [From Vladimir Orlovsky (4vladimir(AT)gmail.com), Oct 30 2008]
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CROSSREFS
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Essentially triangular numbers (A000217) plus 3. Cf. A000124.
Sequence in context: A097557 A123648 A129632 this_sequence A098578 A076968 A098889
Adjacent sequences: A016025 A016026 A016027 this_sequence A016029 A016030 A016031
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KEYWORD
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nonn
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AUTHOR
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Robert G. Wilson v (rgwv(AT)rgwv.com)
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