experimental::YangBaxterGraph – domain for Yang-Baxter graphs
experimental::YangBaxterGraph (top, operators SpectralParameters=parameters Values = values) represents the Yang Baxter graph with top element top, and operators operators.
Details:
Let be some object, and be operators that can be repeatedly applied on . Each sequence , gives thus rise to an object that we write as (action on the right, most of the time to model an action on positions). We further assume that the operators satisfy the braid relations and for , as well as quadratic relations such as . In that case, the image can essentially be indexed by the permutation , where denotes the elementary transposition. We denote this image by .
The Yang-Baxter graph is the graph whose vertices are labelled by the images , and whose edges are labelled by the operators.
In most input and output methods of this domain, permutations are represented by their lexicographically minimal reduced word (see combinat::permutations::reducedWordLexMin); some of the methods do not require the input word to be reduced.
The spectral parameters allow to deform the operators. Namely, the image of by the operator is given by .
The value parameter values allows for restricting to an interval of the full permutahedron, in order to take the symmetries of values into account. Namely, is only computed if is, and the transposition makes the word strictly smaller (lexicographically?).
As a first example, we construct the Yang-Baxter graph with top element [a,b,c], and the two elementary transpositions s1 and s2 as operators. The function makeOperator is used to build those operators:
makeOperator :=
proc(i)
option escape;
begin
w -> combinat::words::swap(w,i);
end:
s1 := makeOperator(1):
s2 := makeOperator(2):
YB := experimental::YangBaxterGraph([a,b,c], [s1,s2]):
s1 and s2 are just usual MuPAD functions. For example:
s1([a, b, c])
They furthermore satisfy the braid relations:
s1(s2(s1([a, b, c]))) = s2(s1(s2([a, b, c])))
Altogether, the vertices of the Yang-Baxter graph are simply all permutations of the list [a,b,c]:
YB::basis::list()
Background:
Reference: lectures notes of Alain Lascoux, http://phalanstere.univ-mlv.fr/ al/. FIXME.
Changes in MuPAD 3.1
New Function.