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 A185355 Number of n X n symmetric (0,1)-matrices containing four ones. 1
 0, 1, 12, 52, 150, 345, 686, 1232, 2052, 3225, 4840, 6996, 9802, 13377, 17850, 23360, 30056, 38097, 47652, 58900, 72030, 87241, 104742, 124752, 147500, 173225, 202176, 234612, 270802, 311025, 355570, 404736, 458832, 518177, 583100, 653940, 731046, 814777 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,3 COMMENTS Based on equation (11) from the Cameron et al., reference. LINKS G. C. Greubel, Table of n, a(n) for n = 1..1000 P. Cameron, T. Prellberg and D. Stark, Asymptotics for incidence matrix classes, Electron. J. Combin. 13 (2006), #R85, p. 11. FORMULA a(n) = Sum_{k=0..2} C(C(n,2),k)*C(n,4-2*k). a(n) = n^2*(n-1)*(5*n-7)/12. G.f.: x^2*(1+7*x+2*x^2)/(1-x)^5. MAPLE a:= n-> (7+(5*n-12)*n)*n^2/12: seq (a(n), n=1..40); MATHEMATICA Table[n^2*(n - 1)*(5*n - 7)/12, {n, 1, 50}] (* G. C. Greubel, Jun 28 2017 *) PROG (PARI) for(n=1, 25, print1(n^2*(n-1)*(5*n-7)/12, ", ")) \\ G. C. Greubel, Jun 28 2017 CROSSREFS Cf. A002378, A006331. Column m=4 of A184948. Sequence in context: A045219 A325379 A054410 * A297757 A223249 A195544 Adjacent sequences:  A185352 A185353 A185354 * A185356 A185357 A185358 KEYWORD nonn AUTHOR L. Edson Jeffery, Feb 29 2012 STATUS approved

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Last modified July 22 06:33 EDT 2019. Contains 325216 sequences. (Running on oeis4.)