Cycle (mathematics)

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This article is about group theory. For cycles in homological algebra, see Chain complex#Fundamental terminology. For cycles in graph theory, see Cycle (graph theory).

In mathematics, and in particular in group theory, a cycle is a permutation of the elements of some set X which maps the elements of some subset S of X to each other in a cyclic fashion, while fixing (i.e., mapping to themselves) all other elements of X. For example, the permutation of {1, 2, 3, 4} that sends 1 to 3, 3 to 2, 2 to 4 and 4 to 1 is a cycle, while the permutation that sends 1 to 3, 3 to 1, 2 to 4 and 4 to 2 is not (it separately permutes the pairs {1, 3} and {2, 4}). The set S is called the orbit of the cycle.

Definition[edit]

A permutation of a set X, which is a bijective function \sigma:X\to X, is called a cycle if the action on X of the subgroup generated by \sigma has at most one orbit with more than a single element.[1] This notion is most commonly used when X is a finite set; then of course the largest orbit, S, is also finite. Let s_0 be any element of S, and put s_i=\sigma^i(s_0) \, for any i\in\mathbf{Z}. If S is finite, there is a minimal number k \geq 1 for which s_k=s_0. Then S=\{ s_0, s_1, \ldots, s_{k-1}\}, and \sigma is the permutation defined by

\sigma(s_i) = s_{i+1} \quad\mbox{for }0\leq i<k

and \sigma(x)=x for any element of X\setminus S. The elements not fixed by \sigma can be pictured as

s_0\mapsto s_1\mapsto s_2\mapsto\cdots\mapsto s_{k-1}\mapsto s_k=s_0.

A cycle can be written using the compact cycle notation \sigma = (s_0~s_1~\dots~s_{k-1}) (there are no commas between elements in this notation, to avoid confusion with a k-tuple). The length of a cycle, is the number of elements of its largest orbit. A cycle of length k is also called a k-cycle.

The orbit of a 1-cycle is called a fixed point of the permutation, but as a permutation every 1-cycle is the identity permutation.[2] When cycle notation is used, the 1-cycles are often suppressed when no confusion will result.[3]

Basic properties[edit]

One of the basic results on symmetric groups says that any permutation can be expressed as the product of disjoint cycles (more precisely: cycles with disjoint orbits); such cycles commute with each other, and the expression of the permutation is unique up to the order of the cycles (but note that the cycle notation is not unique: each k-cycle can itself be written in k different ways, depending on the choice of s_0 in its orbit).[4] The multiset of lengths of the cycles in this expression (the cycle type) is therefore uniquely determined by the permutation, and both the signature and the conjugacy class of the permutation in the symmetric group are determined by it.[5]

The number of k-cycles in the symmetric group Sn is given, for 1\leq k\leq n, by the following equivalent formulas

\binom nk(k-1)!=\frac{n(n-1)\cdots(n-k+1)}{k}=\frac{n!}{(n-k)!k}

A k-cycle has signature (−1)k − 1.

Transpositions[edit]

Array of transpositions

A cycle with only two elements is called a transposition. For example, the permutation of {1, 2, 3, 4} that sends 1 to 1, 2 to 4, 3 to 3 and 4 to 2 is a transposition (specifically, the transposition that swaps 2 and 4).

Properties[edit]

Any permutation can be expressed as the composition (product) of transpositions—formally, they are generators for the group.[6] In fact, if one takes a=1, b=2, ..., e=5, then any permutation can be expressed as a product of adjacent transpositions, meaning the transpositions (k~~k+1), in this case (1~2), (2~3), (3~4), and (4~5). This follows because an arbitrary transposition can be expressed as the product of adjacent transpositions. Concretely, one can express the transposition (k~~l) where k < l by moving k to l one step at a time, then moving l back to where k was, which interchanges these two and makes no other changes:

(k~~l) = (k~~k+1)\cdot(k+1~~k+2)\cdots(l-1~~l)\cdot(l-2~~l-1)\cdots(k~~k+1).

The decomposition of a permutation into a product of transpositions is obtained for example by writing the permutation as a product of disjoint cycles, and then splitting iteratively each of the cycles of length 3 and longer into a product of a transposition and a cycle of length one less:

(a,b,c,d,\ldots,y,z) = (a,b)\cdot (b,c,d,\ldots y,z)

This means the initial request is to move a to b, b to c, y to z and finally z to a. Instead one may roll the elements keeping a where it is by executing the right factor first (as usual in operator notation, and following the convention in the article on Permutations). This has moved z to the position of b, so after the first permutation, the elements a and z are not yet at their final positions. The transposition (a,b), executed thereafter, then addresses z by the index of b to swap what initially were a and z.

In fact, the symmetric group is a Coxeter group, meaning that it is generated by elements of order 2 (the adjacent transpositions), and all relations are of a certain form.

One of the main results on symmetric groups states that either all of the decompositions of a given permutation into transpositions have an even number of transpositions, or they all have an odd number of transpositions.[7] This permits the parity of a permutation to be a well-defined concept.

See also[edit]

Notes[edit]

  1. ^ Fraleigh 1993, p. 103
  2. ^ Rotman 2006, p. 108
  3. ^ Sagan 1991, p. 2
  4. ^ To be technically correct a factorization should contain one 1-cycle for each fixed point of the permutation. See Rotman (2006, pp. 113-114).
  5. ^ Rotman 2006, p. 117, 121
  6. ^ Rotman 2006, p. 118, Prop. 2.35
  7. ^ Rotman 2006, p. 122

References[edit]

  • Anderson, Marlow and Feil, Todd (2005), A First Course in Abstract Algebra, Chapman & Hall/CRC; 2nd edition. ISBN 1-58488-515-7.
  • Fraleigh, John (1993), A first course in abstract algebra (5th ed.), Addison Wesley, ISBN 978-0-201-53467-2 
  • Rotman, Joseph J. (2006), A First Course in Abstract Algebra with Applications (3rd ed.), Prentice-Hall, ISBN 978-0-13-186267-8 
  • Sagan, Bruce E. (1991), The Symmetric Group / Representations, Combinatorial Algorithms & Symmetric Functions, Wadsworth & Brooks/Cole, ISBN 978-0-534-15540-7 

External links[edit]

This article incorporates material from cycle on PlanetMath, which is licensed under the Creative Commons Attribution/Share-Alike License.