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A016028 Expansion of (1 - x + x^4) / (1 - x)^3. 3
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; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

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 Janjic, Dec 28 2007

LINKS

Table of n, a(n) for n=1..51.

R. Aharoni and E. Berger, The number of edges in critical strongly connected graphs, arXiv:math/9911113 [math.CO], 1999.

Index entries for linear recurrences with constant coefficients, signature (3,-3,1).

FORMULA

Also, from the third term on, triangular numbers + 3. - Alexandre Wajnberg, Dec 10 2005

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

MATHEMATICA

i=0; s=3; lst={1, 2}; Do[s+=n+i; AppendTo[lst, s], {n, 0, 6!, 1}]; lst (* Vladimir Joseph Stephan Orlovsky, Oct 30 2008 *)

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 *)

PROG

(PARI) Vec((1-x+x^4)/(1-x)^3+O(x^99)) \\ Charles R Greathouse IV, Sep 25 2012

CROSSREFS

Essentially triangular numbers (A000217) plus 3. Cf. A000124.

Sequence in context: A286929 A255525 A129632 * A239551 A219282 A098578

Adjacent sequences:  A016025 A016026 A016027 * A016029 A016030 A016031

KEYWORD

nonn,easy

AUTHOR

Robert G. Wilson v

EXTENSIONS

Definition corrected by Harvey P. Dale, Nov 30 2015

STATUS

approved

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Last modified May 23 06:42 EDT 2017. Contains 286909 sequences.