More precisely, the opposite of a ring (R, +, ·) is the ring (R, +, ∗) whose multiplication ∗ is defined by a ∗ b = b · a. Ring addition is per definition commutative.
Two rings R1 and R2 are isomorphic if and only if their corresponding opposite rings are isomorphic. The opposite of the opposite of a ring is isomorphic to that ring. A ring and its opposite ring are anti-isomorphic.
A commutative ring is always equal to its opposite ring. A non-commutative ring may or may not be isomorphic to its opposite ring.
- Berrick & Keating (2000), p. 19
- Berrick, A. J.; Keating, M. E. (2000). An Introduction to Rings and Modules With K-theory in View. Cambridge studies in advanced mathematics. 65. Cambridge University Press. ISBN 978-0-521-63274-4.
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