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# Algebra over a field

If K is a field, then an algebra over K, or a K-algebra, is a vector space A over K equipped with a compatible notion of multiplication. A straightforward generalisation allows K to be any commutative ring.

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To be precise, let K be a field, and let A be a vector space over K. Suppose we are given a binary operation A×AA, with the result of this operation applied to the vectors x and y in A written as xy. Suppose further that:

• (x + y)z = xz + yz;
• x(y + z) = xy + xz;
• (ax)y = a(xy); and
• x(by) = b(xy);
for all scalars a and b in K and all vectors x, y, and z. Then with this operation, A becomes an algebra over K, and K is the base field of A. If the multiplication is commutative, then A is called a commutative algebra.

In general, xy is the product of x and y, and the operation is called vector multiplication. However, several special kinds of algebras go by different names.

Algebras can also more generally be defined over any commutative ring K: we need a module A over K and a multiplication operation which satisfies the same identities as above; then A is a K-algebra, and K is the base ring of A.

Vector multiplication is a bilinear operator from A × A to A, and is therefore completely determined by the multiplication of basis elements of A. Conversely, once a basis for A has been choses, the products of basis elements can be set arbitrarily, and then extended in a unique way to a bilinear operator on A, i.e. so that the resulting multiplication will satisfy the algebra laws.

Thus, given the field K, any algebra can be specified up to isomorphism by giving its dimension (say n), and specifying n3 structure coefficients ci,j,k, which are scalars. These structure coefficients determine the multiplication in A via the following rule:

$\mathbf{e}_{i} \mathbf{e}_{j} = \sum_{k=1}^n c_{i,j,k} \mathbf{e}_{k}$
where e1,...,en form a basis of A. The only requirement on the structure coefficients is that, if the dimenion n is an infinite number, then this sum must always converge (in whatever sense is appropriate for the situation).

In mathematical physics, the structure coefficients are often written ci,jk, and their defining rule is written using the Einstein summation convention as

eiej = ci,jkek.
If you apply this to vectors written in index notation[?], then this becomes
(xy)k = ci,jkxiyj.

If K is only a commutative ring and not a field, then the same process works if A is a free module[?] over K. If it isn't, then the multiplication is still completely determined by its action on a generating set[?] of A; however, the structure constants can't be specified arbitrarily in this case.

The most important types of algebras are:

In geometric quantisation[?], one considers Poisson algebras[?], which carry two multiplications, turning them into commutative algebras and Lie algebras in different ways.

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