A209979 Number of unimodular 2 X 2 matrices having all elements in {1,2,...,n}.

0, 0, 4, 16, 28, 56, 68, 112, 140, 184, 212, 288, 316, 408, 452, 512, 572, 696, 740, 880, 940, 1032, 1108, 1280, 1340, 1496, 1588, 1728, 1820, 2040, 2100, 2336, 2460, 2616, 2740, 2928, 3020, 3304, 3444, 3632, 3756, 4072, 4164, 4496, 4652, 4840

Offset: 0

Author

Clark Kimberling, Mar 16 2012

Keywords

• nonn

Comments

Equivalently, the number of 2 X 2 matrices having all elements in {1,2,...,n} and having an inverse whose elements are all integers.

See A210000 for a guide to related sequences.

Links

• Andrew Howroyd, Table of n, a(n) for n = 0..1000

Crossrefs

Cf. A196227, A209978, A210000.

Programs

Mathematica

(See the Mathematica section at A210000.)

Formula

a(n) = 2*A196227(n) = 4*A209978(n). - Andrew Howroyd, May 05 2020