In mathematics, a shuffle algebra is a Hopf algebra with a basis corresponding to words on some set, whose product is given by the shuffle product XшY of two words X, Y: the sum of all ways of interlacing them. The interlacing is given by the riffle shuffle permutation.
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The shuffle algebra on a finite set is the graded dual of the universal enveloping algebra of the free Lie algebra on the set.
Over the rational numbers, the shuffle algebra is isomorphic to the polynomial algebra in the Lyndon words.
Shuffle product
The shuffle product of words of lengths m and n is a sum over the (m+n)!/m!n! ways of interleaving the two words, as shown in the following examples:
ab ш xy = abxy + axby + xaby + axyb + xayb + xyab ; aaa ш aa = 10aaaaa .It may be defined inductively by
ua ш vb = (u ш vb)a + (ua ш v)b .The shuffle product was introduced by Eilenberg & Mac Lane (1953). The name "shuffle product" refers to the fact that the product can be thought of as a sum over all ways of riffle shuffling two words together: this is the riffle shuffle permutation. The product is commutative and associative.
The shuffle product of two words in some alphabet is often denoted by the shuffle product symbol ш (a cyrillic sha, or the unicode character SHUFFLE PRODUCT (U+29E2)).
Infiltration product
The closely related infiltration product was introduced by Chen, Fox & Lyndon (1958). It is defined inductively on words over an alphabet A by
fa ↑ ga = (f↑ga)a + (fa↑g)a + (f↑g)a ; fa ↑ gb = (f↑gb)a + (fa↑g)b .For example:
ab ↑ ab = ab + 2aab + 2abb + 4 aabb + 2abab ; ab ↑ ba = aba + bab + abab + 2abba + 2baab + baba .The infiltration product is also commutative and associative.