The algebra of square matrices of size n 2 over the field of complex
numbers is, evidently, the best-known example of a non-commutative alge-
1 bra - Subalgebras and subrings of this algebra (for example, the ring
of n x n matrices with integral entries) arise naturally in many areas
of mathemat- ics. Historically however, the study of matrix algebras was
preceded by the discovery of quatemions which, introduced in 1843 by
Hamilton, found ap- plications in the classical mechanics of the past
century. Later it turned out that quaternion analysis had important
applications in field theory. The al- gebra of quaternions has become
one of the classical mathematical objects; it is used, for instance, in
algebra, geometry and topology. We will briefly focus on other examples
of non-commutative rings and algebras which arise naturally in
mathematics and in mathematical physics. The exterior algebra (or
Grassmann algebra) is widely used in differential geometry - for
example, in geometric theory of integration. Clifford algebras, which
include exterior algebras as a special case, have applications in rep-
resentation theory and in algebraic topology. The Weyl algebra (Le.
algebra of differential operators with- polynomial coefficients) often
appears in the representation theory of Lie algebras. In recent years
modules over the Weyl algebra and sheaves of such modules became the
foundation of the so-called microlocal analysis. The theory of operator
algebras (Le.