OFFSET
1,1
COMMENTS
Possibly this sequence gives the number of 2 X 2 matrices with all terms in {0,1,...,n} and positive determinant, as evidenced by a program in the Mathematica section. - Clark Kimberling, Mar 19 2012
LINKS
R. H. Hardin, Table of n, a(n) for n = 1..200
EXAMPLE
Some solutions for n=2:
..1..0....1..0....1..2....0..2....1..1....1..1....0..1....2..0....0..2....1..2
..2..2....1..2....2..1....1..0....2..0....1..2....2..0....2..1....1..2....2..2
Contribution from Clark Kimberling, Mar 19 2012: (Start)
As an example for counting positive determinants (see Comments), the 3 matrices counted by a(1) are
1 0.....1 1.....1 0
0 1.....0 1.....1 1 (End)
MATHEMATICA
a = 0; b = n; z1 = 45;
t[n_] := t[n] = Flatten[Table[w*z - x*y, {w, a, b}, {x, a, b}, {y, a, b}, {z, a, b}]]
c[n_, k_] := c[n, k] = Count[t[n], k]
c1[n_, m_] := c1[n, m] = Sum[c[n, k], {k, 0, m}]
Table[c1[n, n^2] - c[n, 0], {n, 0, z1}]
(* Clark Kimberling, Mar 19 2012 *)
CROSSREFS
KEYWORD
nonn
AUTHOR
R. H. Hardin, Jan 06 2011
STATUS
approved