A linear operator is called a divided difference operator if it is not identically trivial and satisfies the following identity
(12) |
for all , where is not invertible in .
An algebra over additively isomorphic to is called an Ore extension if and
(19) |
The operator from ( 19 ) is called Oreβs -derivation.
A trialgebra with and associative products on (where may be partially defined, only) and a coassociative coproduct on is given if both and are bialgebras and the following compatibility condition between the products is satisfied for arbitrary elements :
whenever both sides are defined.
For two mapping classes , we let
Let , where is canonically identified with , be the left modular function defined by
(29) |
where is the counit of defined by for every .