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
John Cerkan, Table of n, a(n) for n = 0..445
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
a(0) = 0, a(1) = 1, a(n) = (n-2) * a(n-1) + A000985(n - 2), n >= 2. - Sean A. Irvine, Sep 24 2015
MATHEMATICA
(* b = A000985 *) Clear[a, b]; b[0] = 1; b[1] = 1; b[2] = 3; b[3] = 11; b[4] = 56; b[n_] := b[n] = (2*n-1)*b[n-1] - (n-2)*(n-1)*b[n-2] - (n-2)*(n - 1)*b[n-3] + (n-3)*(n-2)*(n-1)*b[(n-4)]/2; a[0] = 0; a[1] = 1; a[n_] := a[n] = (n-2)*a[n-1] + b[n-2]; Array[a, 22, 0] (* Jean-François Alcover, Feb 15 2016, after Sean A. Irvine *)
CROSSREFS
KEYWORD
nonn
AUTHOR
EXTENSIONS
More terms from Sean A. Irvine, Sep 24 2015
STATUS
approved