A minimal homogeneous generating system of the algebra of semi-invariants of
tuples of two-by-two matrices over an infinite field of characteristic two or
over the ring of integers is given. In an alternative interpretation this
yields a minimal system of homogeneous generators for the vector invariants of
the special orthogonal group of degree four over a field of characteristic two
or over the ring of integers. An irredundant separating system of
semi-invariants of tuples of two-by-two matrices is also determined, it turns
out to be independent of the characteristic.Comment: A crucial reference to a paper of A. Lopatin was adde
