OFFSET
0,3
COMMENTS
It is also the number of 2 X 2 integer matrices with elements from {1,...,n} whose determinant is -2.
LINKS
Chai Wah Wu, Table of n, a(n) for n = 0..10000
FORMULA
a(n) = A209973(n) - 4*n - 2 for n >= 2. - Chai Wah Wu, Nov 28 2016
MATHEMATICA
Table[cnt = 0; Do[If[a*d-b*c == 2, cnt++], {a, n}, {b, n}, {c, n}, {d, n}]; cnt, {n, 50}] (* T. D. Noe, Oct 11 2011 *)
CROSSREFS
KEYWORD
nonn
AUTHOR
Aldo González Lorenzo, Oct 10 2011
EXTENSIONS
a(0) added by Chai Wah Wu, Nov 27 2016
STATUS
approved