Utente:Grasso Luigi/sandbox4/Parola (Teoria dei gruppi)

Nella teoria dei gruppi, una parola è qualsiasi composizione di elementi e dei rispettivi inversi di un gruppo.

Per esempio, se consideriamo un insieme alfabeto S = {x, y, z}⊆G allora xy, z⁻¹xzz e y⁻¹zxx⁻¹yz⁻¹ sono esempi di parole.

Due parole differenti potrebbero dare lo stesso valore in G, o anche in ogni gruppo. Le parole svolgono un ruolo importante nella teoria dei gruppi liberi e nella presentazione di un gruppo, e sono oggetti centrali di studio nella teoria combinatoria dei gruppi.

Definizioni

Let G be a group, and let S be a subset of G. A word in S is any expression of the form

$s_{1}^{\varepsilon _{1}}s_{2}^{\varepsilon _{2}}\cdots s_{n}^{\varepsilon _{n}}$

where s₁,...,s_n are elements of S, called generators, and each ε_i is ±1. The number n is known as the length of the word.

Each word in S represents an element of G, namely the product of the expression. By convention, the unique identity element can be represented by the empty word, which is the unique word of length zero.

Notazione

When writing words, it is common to use exponential notation as an abbreviation. For example, the word

$xxy^{-1}zyzzzx^{-1}x^{-1}\,$

could be written as

$x^{2}y^{-1}zyz^{3}x^{-2}.\,$

This latter expression is not a word itself—it is simply a shorter notation for the original.

When dealing with long words, it can be helpful to use an overline to denote inverses of elements of S. Using overline notation, the above word would be written as follows:

$x^{2}{\overline {y}}zyz^{3}{\overline {x}}^{2}.\,$

Parole ridotte

Any word in which a generator appears next to its own inverse (xx⁻¹ or x⁻¹x) can be simplified by omitting the redundant pair:

$y^{-1}zxx^{-1}y\;\;\longrightarrow \;\;y^{-1}zy.$

This operation is known as reduction, and it does not change the group element represented by the word. Reductions can be thought of as relations (defined below) that follow from the group axioms.

A reduced word is a word that contains no redundant pairs. Any word can be simplified to a reduced word by performing a sequence of reductions:

$xzy^{-1}xx^{-1}yz^{-1}zz^{-1}yz\;\;\longrightarrow \;\;xyz.$

The result does not depend on the order in which the reductions are performed.

A word is cyclically reduced if and only if every cyclic permutation of the word is reduced.

Operazioni sulle parole

The product of two words is obtained by concatenation:

$\left(xzyz^{-1}\right)\left(zy^{-1}x^{-1}y\right)=xzyz^{-1}zy^{-1}x^{-1}y.$

Even if the two words are reduced, the product may not be.

The inverse of a word is obtained by inverting each generator, and reversing the order of the elements:

$\left(zy^{-1}x^{-1}y\right)^{-1}=y^{-1}xyz^{-1}.$

The product of a word with its inverse can be reduced to the empty word:

$zy^{-1}x^{-1}y\;y^{-1}xyz^{-1}=1.$

You can move a generator from the beginning to the end of a word by conjugation:

$x^{-1}\left(xy^{-1}z^{-1}yz\right)x=y^{-1}z^{-1}yzx.$

Insieme di elementi generatori

A subset S of a group G is called a generating set if every element of G can be represented by a word in S.

When S is not a generating set for G, the set of elements represented by words in S is a subgroup of G, known as the subgroup of G generated by S and usually denoted $\langle S\rangle$. It is the smallest subgroup of G that contains the elements of S.

Forme normali

A normal form for a group G with generating set S is a choice of one reduced word in S for each element of G. For example:

• The words 1, i, j, ij are a normal form for the Klein four-group with Template:Math}  and 1 representing the empty word (the identity element for the group).
• The words 1, r, r², ..., r^n-1, s, sr, ..., sr^n-1 are a normal form for the dihedral group Dih_n with Template:Math}  and 1 as above.
• The set of words of the form x^myⁿ for m,n ∈ Z are a normal form for the direct product of the cyclic groups Template:Angbr and Template:Angbr with Template:Math}.
• The set of reduced words in S are the unique normal form for the free group over S.

Insieme di relazioni

If S is a generating set for a group G, a relation is a pair of words in S that represent the same element of G. These are usually written as equations, e.g. $x^{-1}yx=y^{2}.\,$ A set ${\mathcal {R}}$ of relations defines G if every relation in G follows logically from those in ${\mathcal {R}}$ using the axioms for a group. A presentation for G is a pair $\langle S\mid {\mathcal {R}}\rangle$, where S is a generating set for G and ${\mathcal {R}}$ is a defining set of relations.

For example, the Klein four-group can be defined by the presentation

$\langle i,j\mid i^{2}=1,\,j^{2}=1,\,ij=ji\rangle .$

Here 1 denotes the empty word, which represents the identity element.

Gruppi liberi

If S is any set, the free group over S is the group with presentation $\langle S\mid \;\rangle$. That is, the free group over S is the group generated by the elements of S, with no extra relations. Every element of the free group can be written uniquely as a reduced word in S.

Bibliografia

• David Epstein, Word Processing in Groups, AK Peters, 1992, ISBN 0-86720-244-0..
• (RU) On the algorithmic unsolvability of the word problem in group theory, in Trudy Mat. Inst. Steklov, vol. 44, 1955, pp. 1–143.
• Robinson, Derek John Scott, A course in the theory of groups, Berlin, Springer-Verlag, 1996, ISBN 0-387-94461-3.
• Rotman, Joseph J., An introduction to the theory of groups, Berlin, Springer-Verlag, 1995, ISBN 0-387-94285-8.
• Combinatorial group theory, Berlin, Springer, 2001, ISBN 3-540-41158-5.
• Combinatorial group theory: presentations of groups in terms of generators and relations, New York, Dover, 2004, ISBN 0-486-43830-9.
• Stillwell, John, Classical topology and combinatorial group theory, Berlin, Springer-Verlag, 1993, ISBN 0-387-97970-0.

Voci correlate

• Word problem (mathematics)
• Word problem for groups