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 A001495 Number of symmetric 0-1 (n+1) X (n+1) matrices with row sums 2 and first row starting 1,1 for n>0, a(0)=1. (Formerly M2947 N1188) 1
 1, 1, 1, 3, 13, 70, 462, 3592, 32056, 322626, 3611890, 44491654, 597714474, 8693651092, 136059119332, 2279212812480, 40681707637888, 770631412413148, 15438647456063004, 326091322648369684, 7241563996136849260, 168657537987709667976, 4110364564664358194536 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,4 REFERENCES 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). LINKS Alois P. Heinz, Table of n, a(n) for n = 0..200 H. Gupta, Enumeration of symmetric matrices, Duke Math. J., 35 (1968), vol 3, 653-659. H. Gupta, >Enumeration of symmetric matrices (annotated scanned copy) FORMULA It appears that E.g.f. = 1+int((1-t)^(-3/2)*exp( t*(t^2+3*t-2)/(4-4*t) ),t = 0..x). - Mark van Hoeij, Oct 25 2011 Recursion: a(n) = (n-1) a(n-1) + (n-2)^2 a(n-2) - (n-2)(n-3)(n-4) a(n-3) - (1/2) (n-2)(n-3)(n-4) a(n-4) - (1/2)(n-2)(n-3)(n-4)(n-5) a(n-5). - Robert Israel, Aug 05 2013 a(n) ~ exp(sqrt(2*n)-n-3/2) * n^(n-1/2) * (1+31/(24*sqrt(2*n))). - Vaclav Kotesovec, Aug 14 2013 EXAMPLE a(3) = 3 because there are 3 symmetric 4 X 4 0-1 matrices with row sums 2 and first row 1 1 0 0, namely 1100   1100       1100 1001   1010       1100 0011   0101       0011 0110,  0011  and  0011. MAPLE a:= proc(n) a(n):= `if`(n<2, 1, (n-1) *a(n-1) +(n-2)^2 *a(n-2) -        (n-2)*(n-3)*(n-4)* a(n-3) - (1/2)* (n-2)*(n-3)*(n-4)* a(n-4) -        (1/2)*(n-2)*(n-3)*(n-4)*(n-5)* a(n-5))     end: seq(a(n), n=0..30);  # Robert Israel, Aug 05 2013 MATHEMATICA max = 30; egf = 1 + Integrate[(1-t)^(-3/2)*Exp[t (t^2 + 3 t - 2)/(4 - 4 t)] + O[t]^max // Normal, t]; CoefficientList[egf, t]* Range[0, max]! (* Jean-François Alcover, Apr 06 2017, after Mark van Hoeij *) CROSSREFS Sequence in context: A154677 A121586 A024337 * A284217 A213690 A227117 Adjacent sequences:  A001492 A001493 A001494 * A001496 A001497 A001498 KEYWORD nonn AUTHOR EXTENSIONS Better name from and edited by Robert Israel, Aug 05 2013 STATUS approved

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Last modified January 22 15:57 EST 2019. Contains 319364 sequences. (Running on oeis4.)