order in an algebra
Let be an algebra (not necessarily commutative), finitely generated
over. An order of is a subringof which is finitely generated as a-module and which satisfies .
Examples:
- 1.
The ring of integers
in a number field is an order, known asthe maximal order
.
- 2.
Let be a quadratic imaginary field and itsring of integers. For each integer the ring is an order of (in fact it can beproved that every order of is of this form). The number is called the of the order .
Reference: Joseph H. Silverman, The arithmetic ofelliptic curves, Springer-Verlag, New York, 1986.