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 A135593 Number of n X n symmetric (0,1)-matrices with exactly n+1 entries equal to 1 and no zero rows or columns. 1
 2, 9, 36, 140, 540, 2142, 8624, 35856, 152280, 666380, 2982672, 13716144, 64487696, 310693320, 1528801920, 7691652992, 39474925344, 206758346256, 1103332900160, 5999356762560, 33197323465152, 186925844947424, 1069977071943936 (list; graph; refs; listen; history; text; internal format)
 OFFSET 2,1 LINKS Vincenzo Librandi, Table of n, a(n) for n = 2..200 FORMULA E.g.f.: x^2*(x+2)/2*exp(x*(x+2)/2). Recurrence (for n>5): (n-5)*(n-2)*a(n) = (n-6)*n*a(n-1) + (n-4)*(n-1)*n*a(n-2). - Vaclav Kotesovec, Oct 20 2012 a(n) ~ 1/4*sqrt(2)*exp(sqrt(n)-n/2-1/4)*n^(n/2+3/2). - Vaclav Kotesovec, Oct 20 2012 MAPLE A135593 := proc(n) n!*coeftayl( x^2*(x+2)/2*exp(x*(x+2)/2), x=0, n) ; end: seq(A135593(n), n=2..40) ; # R. J. Mathar, Mar 31 2008 MATHEMATICA Rest[Rest[CoefficientList[Series[x^2*(x+2)/2*E^(x*(x+2)/2), {x, 0, 20}], x]* Range[0, 20]!]] (* Vaclav Kotesovec, Oct 20 2012 *) Flatten[{2, 9, RecurrenceTable[{(n-5)*(n-2)*a[n]==(n-6)*n*a[n-1]+(n-4)*(n-1)*n*a[n-2], a[4]==36, a[5]==140}, a, {n, 4, 20}]}] (* Vaclav Kotesovec, Oct 20 2012 *) CROSSREFS Cf. A000085, A055602, A086364. Sequence in context: A288834 A134759 A007946 * A027995 A077836 A003125 Adjacent sequences: A135590 A135591 A135592 * A135594 A135595 A135596 KEYWORD easy,nonn AUTHOR Vladeta Jovovic, Feb 25 2008 EXTENSIONS More terms from R. J. Mathar, Mar 31 2008 STATUS approved

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Last modified September 15 18:36 EDT 2024. Contains 375954 sequences. (Running on oeis4.)