login
The OEIS is supported by the many generous donors to the OEIS Foundation.

 

Logo
Hints
(Greetings from The On-Line Encyclopedia of Integer Sequences!)
A001205 Number of clouds with n points; number of undirected 2-regular labeled graphs; or number of n X n symmetric matrices with (0,1) entries, trace 0 and all row sums 2.
(Formerly M2937 N1181)
23
1, 0, 0, 1, 3, 12, 70, 465, 3507, 30016, 286884, 3026655, 34944085, 438263364, 5933502822, 86248951243, 1339751921865, 22148051088480, 388246725873208, 7193423109763089, 140462355821628771, 2883013994348484940 (list; graph; refs; listen; history; text; internal format)
OFFSET

0,5

COMMENTS

a(n) is the number of ways of covering K_n with cycles of length >= 3. Also number of 'frames' on n lines: given n lines in general position (none parallel and no three concurrent), a frame is a subset of n of the e C(n,2) points of intersection such that no three points are on the same line. - Mitch Harris, Jul 06 2006

REFERENCES

Miklos Bona, editor, Handbook of Enumerative Combinatorics, CRC Press, 2015, p. 410-411.

L. Comtet, Advanced Combinatorics, Reidel, 1974, p. 276 and 279.

S. R. Finch, Mathematical Constants, Cambridge, 2003, Section 5.6.7.

Ph. Flajolet, Singular combinatorics, pp. 561-571, Proc. Internat. Congr. Math., Beijing 2002, Higher Education Press, Beijing, 2002, Vol III.

I. P. Goulden and D. M. Jackson, Combinatorial Enumeration, John Wiley and Sons, N.Y., 1983, ex. 3.3.6b, 3.3.34.

N. J. A. Sloane, A Handbook of Integer Sequences, Academic Press, 1973 (includes this sequence).

N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).

R. P. Stanley, Enumerative Combinatorics, Cambridge, Vol. 2, 1999; see Example 5.2.8. Also problems 5.23 and 5.15(a), case k=3.

Z. Tan and S. Gao, Enumeration of (0,1)-Symmetric Matrices, submitted [From Shanzhen Gao, Jun 05 2009] [apparently unpublished as of 2016]

H. S. Wilf, Generatingfunctionology, Academic Press, NY, 1990, p. 77, Eq. 3.9.1.

W. A. Whitworth, Choice and Chance, Bell, 1901, p. 269, ex. 160.

LINKS

T. D. Noe, Table of n, a(n) for n=0..100

Editorial note, Robinson's constant, Amer. Math. Monthly, 59 (1952), 296-297.

Ph. Flajolet, Singular combinatorics, arXiv:math/0304465 [math.CO], 2003.

Ph. Flajolet and A. Odlyzko, Singularity analysis of generating functions, [Research Report] RR-0826, INRIA. 1988.

Atabey Kaygun, Enumerating Labeled Graphs that Realize a Fixed Degree Sequence, arXiv:2101.02299 [math.CO], 2021.

Rui-Li Liu and Feng-Zhen Zhao, New Sufficient Conditions for Log-Balancedness, With Applications to Combinatorial Sequences, J. Int. Seq., Vol. 21 (2018), Article 18.5.7.

H. Richter, Analyzing coevolutionary games with dynamic fitness landscapes, arXiv preprint arXiv:1603.06374 [q-bio.PE], 2016.

R. Robinson, A new absolute geometric constant?, Amer. Math. Monthly, 58 (1951), 462-469.

Weiping Wang, Tianming Wang, An automatic approach to the generating functions for some special sequences, Ars. Comb. 116 (2018) 263, Example 4.2

H. S. Wilf, Generatingfunctionology, 2nd edn., Academic Press, NY, 1994, p. 86, Eq. 3.9.1.

FORMULA

a(n) ~ n!*exp(-3/4)/sqrt(Pi*n).

E.g.f.: exp(-x/2-x^2/4)/sqrt(1-x).

D-finite with recurrence a(n+1) = n*(a(n)+a(n-2)*(n-1)/2).

1/4^n * Sum_{b=0..floor(n/2)} Sum_{g=0..n-2*b} (-1)^(b+g) * 2^(2b+g) * n! * (2n-4b-2g)! / (b! * g! * (n-2b-g)!^2). - Shanzhen Gao, Jun 05 2009

a(n) = (-1)^n*n!*Sum_{k=0..n}(3/4)^k*binomial(-1/2, n - k)*hypergeom([1/2, -k], [1/2 - n + k], 1/3)/ k!. - Peter Luschny, Aug 26 2017

MAPLE

a := n -> (-1)^n*n!*add((3/4)^k*binomial(-1/2, n-k)*hypergeom([1/2, -k], [1/2-n+k], 1/3)/ k!, k=0..n): seq(simplify(a(n)), n=0..21); # Peter Luschny, Aug 26 2017

MATHEMATICA

m = 21; CoefficientList[ Series[ Exp[-x/2 - x^2/4] / Sqrt[1-x], {x, 0, m}], x]*Table[n!, {n, 0, m}] (* Jean-Fran├žois Alcover, Jun 21 2011, after e.g.f. *)

PROG

(PARI) a(n)=if(n<0, 0, n!*polcoeff(exp(-x/2-x^2/4+x*O(x^n))/sqrt(1-x+x*O(x^n)), n))

(Maxima)

a(n):=sum(sum(binomial(k, i)*binomial(i-1/2, n-k)*(3^(k-i)*n!)/(4^k*k!)*(-1)^(n-i), i, 0, k), k, 0, n);

makelist(a(n), n, 0, 12); /* Emanuele Munarini, Aug 25 2017 */

CROSSREFS

Cf. A000985, A000986, A002137. A diagonal of A059441 and A144163.

Sequence in context: A113341 A125862 A077460 * A346888 A330493 A112320

Adjacent sequences:  A001202 A001203 A001204 * A001206 A001207 A001208

KEYWORD

nonn,easy,nice

AUTHOR

N. J. A. Sloane

STATUS

approved

Lookup | Welcome | Wiki | Register | Music | Plot 2 | Demos | Index | Browse | More | WebCam
Contribute new seq. or comment | Format | Style Sheet | Transforms | Superseeker | Recents
The OEIS Community | Maintained by The OEIS Foundation Inc.

License Agreements, Terms of Use, Privacy Policy. .

Last modified May 23 23:07 EDT 2022. Contains 353993 sequences. (Running on oeis4.)