Idempotent 2-by-2 matrices
(Idempotent) 2-by-2 matrices of determinant 0 and trace 1
over a commutative ring are related to line bundles on two
generators. This is used to describe their orbits under inner (resp.
arbitrary) automorphisms and to construct explicit examples of line
bundles on two generators having infinite order in the Picard group
of the base ring.
Holger P. Petersson < holger.petersson@fernuni-hagen.de >