  A linear group G<GL(n) acts on d-tuples of n x n matrices by simultaneous
conjugation. In [Adv. Math. 19 (1976), 306-381] Procesi established generators
and relations between them for G-invariants, where G is GL(n), O(n), and Sp(n)
and the characteristic of base field is zero. We continue generalization of the
mentioned results to the case of positive characteristic originated by Donkin
in [Invent. Math. 110 (1992), 389-401]. We investigate relations between
generators for O(n)-invariants.
