space and the ring share the same addition operation and are related in certain other ways. An example algebra is the set of 2x2 matrices with realnumbers as entries, with the usual operations of addition and matrix multiplication, and the usual scalar multiplication. Another example is the set of all polynomials with real coefficients, with the usual operations. In more detail, we have: (3) an operation of scalar multiplication, whose input is a scalar and a member of the underlying set and whose output is (4) an operation of addition of members of the underlying set, output is one such member, just as in a vector space or a ring, (5) an operation of multiplication of members of the underlying set, whose input is an ordered pair of such members and whose output is one such member, just as in a ring. This whole thing constitutes an `algebra' iff: (1) it is a vector space if you discard item (5) and (2) it is a ring if you discard (2) and (3) and (3) for any scalar r and any two members A, B of the underlying set we have r(AB) = (rA)B = A(rB). In other words it doesn't matter whether you multiply members of the algebra first and then multiply by the scalar, or multiply one of them by the scalar first and then multiply the two members of the algebra. Note that the A comes before the B because the multiplication is in some cases not commutative, e.g. the matrix example. the usual norm. The multiplication is the operation of multiplication are just what you would expect. [I. N. Herstein, "Topics_in_Algebra"]. (1999-07-14)
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