A pairing can also be considered as an R-linear map, which matches the first definition by setting
.
A pairing is called perfect if the above map is an isomorphism of R-modules.
A pairing is called non-degenerate on the right if for the above map we have that for all implies ; similarly, is called non-degenerate on the left if for all implies .
A pairing is called alternating if and for all m. In particular, this implies , while bilinearity shows . Thus, for an alternating pairing, .
Examples
Any scalar product on a real vector space V is a pairing (set M = N = V, R = R in the above definitions).
The determinant map (2 × 2 matrices over k) → k can be seen as a pairing .
The Hopf map written as is an example of a pairing. For instance, Hardie et al.[1] present an explicit construction of the map using poset models.
For practical purposes, has to be computable in an efficient manner
Note that it is also common in cryptographic literature for all groups to be written in multiplicative notation.
In cases when , the pairing is called symmetric. As is cyclic, the map will be commutative; that is, for any , we have . This is because for a generator , there exist integers , such that and . Therefore .
Slightly different usages of the notion of pairing
Scalar products on complexvector spaces are sometimes called pairings, although they are not bilinear.
For example, in representation theory, one has a scalar product on the characters of complex representations of a finite group which is frequently called character pairing.
^Hardie K.A.1; Vermeulen J.J.C.; Witbooi P.J., A nontrivial pairing of finite T0 spaces, Topology and its Applications, Volume 125, Number 3, 20 November 2002 , pp. 533–542.