A ring in which the equations and with are uniquely solvable. In the case of an associative ring (cf. Associative rings and algebras) it is sufficient to require the existence of a unit 1 and the unique solvability of the equations and for any . A commutative associative skew-field is called a field. An example of a non-commutative associative skew-field is the skew-field of quaternions, defined as the set of matrices of the form
over the field of complex numbers with the usual operations (see Quaternion). An example of a non-associative skew-field is the Cayley–Dickson algebra, consisting of all matrices of the same form as above over the skew-field of quaternions. This skew-field is alternative (see Alternative rings and algebras). Any skew-field is a division algebra either over the field of rational numbers or over a field of residues . The skew-field of quaternions is a -dimensional algebra over the field of real numbers, while the Cayley–Dickson algebra is -dimensional. The dimension of any algebra with division over the field of real numbers is equal to 1, 2, 4, or 8 (see , and also Topological ring). The fields of real or complex numbers and the skew-field of quaternions are the only connected locally compact associative skew-fields (see ). Any finite-dimensional algebra without zero divisors is a skew-field. Any finite associative skew-field is commutative (see , ). An associative skew-field is characterized by the property that any non-zero module over it is free. Any non-associative skew-field is finite-dimensional . A similar result applies to Mal'tsev skew-fields  (see Mal'tsev algebra) and to Jordan skew-fields  (see Jordan algebra). In contrast to the commutative case, not every associative ring without zero divisors can be imbedded in a skew-field (see Imbedding of rings).
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Associative skew-fields are also known as division rings, in particular if they are finite dimensional over their centre. For the imbedding problem see [a1].
The theorem that the only associative division algebras over are , or , the algebra of quaternions, is known as Frobenius' theorem.
|[a1]||P.M. Cohn, "Skew field constructions" , Cambridge Univ. Press (1977)|
|[a2]||P.M. Cohn, "Algebra" , 3 , Wiley (1991) pp. Chapt. 7|
Skew-field. Encyclopedia of Mathematics. URL: http://www.encyclopediaofmath.org/index.php?title=Skew-field&oldid=21398