nLab Sweedler notation




Sweedler notation is a special notation for discussion of operations in coalgebras


If CC is a coassociative coalgebra and then for cCc\in C, the comultiplication Δ\Delta maps cc to an element in CCC\otimes C which is therefore a sum of the form i=1 na ib i\sum_{i=1}^n a_i\otimes b_i. Sweedler suggests that we do not make up new symbols like aa and bb but rather use composed symbols c (1)c_{(1)} and c (2)c_{(2)}. Therefore

Δ(c)= i=1 nc (1)ic (2)i.\Delta(c) = \sum_{i=1}^n c_{(1)i}\otimes c_{(2)i}.

Sweedler notation means that for certain manipulations involving just generic linear operations we actually do not need to think of the summation symbol ii, so we can just write

Δ(c)=c (1)c (2)\Delta(c) = \sum c_{(1)}\otimes c_{(2)}

with or even without summation sign. Surely in either case we need to remember that we do not have a factorization but we do have a sum of possibly more than one entry. One can formalize in fact which manipulations are allowed with such a reduced notation.

It becomes more useful when we take into account coassociativity to justify extending the notation to write

c (1)c (2)c (3):=c (1)(1)c (1)(2)c (2) =c (1)c (2)(1)c (2)(2). \array{\sum c_{(1)}\otimes c_{(2)} \otimes c_{(3)} := \sum c_{(1)(1)}\otimes c_{(1)(2)}\otimes c_{(2)} \\ = \sum c_{(1)}\otimes c_{(2)(1)}\otimes c_{(2)(2)}.}

Furthermore, we can extend it to coactions, e.g. ρ:VVC\rho:V\to V\otimes C, by ρ(v)=v (0)v (1)\rho(v) = \sum v_{(0)}\otimes v_{(1)}. Then we can use the coaction axiom (id VΔ)ρ=(ρid C)ρ(id_V\otimes \Delta)\circ\rho = (\rho\otimes id_C)\circ \rho to write

v (0)v (1)v (2):=v (0)(0)v (0)(1)v (1) =v (0)v (1)(0)v (1)(1), \array{v_{(0)}\otimes v_{(1)} \otimes v_{(2)} := v_{(0)(0)}\otimes v_{(0)(1)} \otimes v_{(1)}\\ = v_{(0)}\otimes v_{(1)(0)}\otimes v_{(1)(1)},}

where we used the sumless Sweedler notation.

One big use is that the scalars like ϵ(a (3))\epsilon(a_{(3)}) can be moved freely along the expression, which is difficult to write without calculating with Sweedler components: one would need lots of brackets and flip operators, and this could be messy and abstract.


The notation is named after Moss Sweedler. Sometimes (though rarely) it is also called Heyneman-Sweedler notation.

Last revised on April 9, 2021 at 20:27:19. See the history of this page for a list of all contributions to it.