

A000241


Crossing number of complete graph with n nodes.
(Formerly M2772 N1115)


10



0, 0, 0, 0, 0, 1, 3, 9, 18, 36, 60, 100, 150, 225, 315, 441, 588
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OFFSET

0,7


COMMENTS

Verified for n=11, 12 by Shengjun Pan and R. Bruce Richter, in "The Crossing Number of K_11 is 100", J. Graph Theory 56 (2) (2007) 128134.
The values for n >= 13 are probably only conjectural.
Also the sum of the dimensions of the irreducible representations of su(3) that first occur in the [n5]th tensor power of the tautological representation.  james dolan (jdolan(AT)math.ucr.edu), Jun 02 2003
It appears that a(n)=C(floor(n/2),2)*C(floor((n1)/2),2). [From Paul Barry, Oct 02 2008]
From Paul Barry, Oct 02 2008: (Start)
We conjecture that this sequence is given by one half of the third coefficient of the denominator polynomial of the nth convergent to the g.f. of n!, in which case the next numbers are 784,1008,1296,1620, 2025, 2475,...
Essentially sum{k=0..n, (1)^(nk) floor(k/2)ceiling(k/2)floor((k1)/2)ceiling((k1)/2)/2}. (End)
One of the most basic questions in knot theory remains unresolved: is crossing number additive under connected sum? In other words, does the equality c(K1#K2) = c(K1) + c(K2) always hold, where c(K) denotes the crossing number of a knot K and K1#K2 is the connected sum of two (oriented) knots K1 and K2? Theorem 1.1. Let K1, . . .,Kn be oriented knots in the 3sphere. Then (c(K1) + . . . + c(Kn)) / 152 <= c(K1# . . . #Kn) <= c(K1) + . . . + c(Kn). [From Jonathan Vos Post, Aug 26 2009]


REFERENCES

Ábrego, Bernardo M.; Aichholzer, Oswin; FernándezMerchant, Silvia; Ramos, Pedro; Salazar, Gelasio. The 2Page Crossing Number of K_n. Discrete Comput. Geom. 49 (2013), no. 4, 747777. MR3068573
JeanPaul Delahaye, in Pour La Science, Feb. 2013, #424, Logique et Calcul. Le problème de la fabrique de briques. (The problem of the brick factory), in French.
P. Erdos and R. K. Guy, Crossing number problems, Amer. Math. Monthly, 80 (1973), 5258.
R. K. Guy, The crossing number of the complete graph, Bull. Malayan Math. Soc., Vol. 7, pp. 6872, 1960.
Kainen, Paul C. On a problem of P. Erdos. J. Combinatorial Theory 51968 374377. MR0231744 (38 #72)
D. McQuillan and R. B. Richter, A parity theorem for drawings of complete ... graphs, Amer. Math. Monthly, 117 (2010), 267273.
A. Owens, On the biplanar crossing number, IEEE Trans. Circuit Theory, 18 (1971), 277280.
Shengjun Pan and R. Bruce Richter, "The Crossing Number of K_11 is 100", J. Graph Theory 56 (2) (2007) 128134.
Saaty, Thomas L., On polynomials and crossing numbers of complete graphs. J. Combinatorial Theory Ser. A 10 (1971), 183184. MR0291013 (45 #107)
T. L. Saaty, The number of intersections in complete graphs, Engrg. Cybernetics 9 (1971), no. 6, 11021104 (1972).; translated from Izv. Akad. Nauk SSSR Tehn. Kibernet. 1971, no. 6, 151154 (Russian). Math. Rev. 58 #21749.
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).
C. Thomassen, Embeddings and minors, pp. 301349 of R. L. Graham et al., eds., Handbook of Combinatorics, MIT Press.


LINKS

Table of n, a(n) for n=0..16.
Drago Bokal, Gasper Fijavz and David R. Wood, The Minor Crossing Number of Graphs with an Excluded Minor, math.CO/0609707.
J. Dolan et al., su(3) and Zarankiewicz's conjecture
Marc Lackenby, The crossing number of composite knots, Aug 25 2009. [From Jonathan Vos Post, Aug 26 2009]
T. L. Saaty, The Minimum Number Of Intersections In Complete Graphs
Eric Weisstein's World of Mathematics, Graph Crossing Number.
Eric Weisstein's World of Mathematics, Guy's Conjecture.
E. Weisstein, Zarankiewicz's Conjecture.html


FORMULA

a(n) ~ n^4/64 (Guy, Kainen)
Empirical g.f.: x^5*(1+x+x^2)/(x+1)^3/(x1)^5, which is the same as the conjectured formula of Guy and Saaty. [Simon Plouffe, Feb 06 2013]


MATHEMATICA

a[n_] := 1/4*Floor[n/2]*Floor[(n1)/2]*Floor[(n2)/2]*Floor[(n3)/2]; Table[a[n], {n, 0, 16}] (* JeanFrançois Alcover, Feb 06 2013, after R.K. Guy and Thomas Saaty's conjectured formula *)


CROSSREFS

It is not known if A000241 and A028723 agree. Cf. A007333, A014540, A030179.
Cf. A121021, A191928.
Sequence in context: A246695 A132920 A127645 * A028723 A213291 A057578
Adjacent sequences: A000238 A000239 A000240 * A000242 A000243 A000244


KEYWORD

nonn,more,nice


AUTHOR

N. J. A. Sloane.


EXTENSIONS

Bokal et al. link from Jonathan Vos Post, Dec 08 2006


STATUS

approved



