# Difference between revisions of "Criterion for element of alternating group to be real"

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# The cycle decomposition has a cycle of even length. | # The cycle decomposition has a cycle of even length. | ||

# The cycle decomposition has two cycles of equal odd length. Note that fixed points are counted as cycles of length 1, so this includes any permutation that has two or more fixed points. | # The cycle decomposition has two cycles of equal odd length. Note that fixed points are counted as cycles of length 1, so this includes any permutation that has two or more fixed points. | ||

− | # All the cycles have distinct odd lengths <math>r_1,r_2,\dots,r_k</math> | + | # All the cycles have distinct odd lengths <math>r_1,r_2,\dots,r_k</math> and <math>\sum_{i=1}^k (r_i - 1)/2</math> is even. In other words, the number of <math>r_i</math>s that are congruent to <math>3</math> modulo <math>4</math> is even. |

Note that the permutations which satisfy condition (1) or (2) are precisely those whose conjugacy class is unsplit from the symmetric group, where every element is real. (see [[splitting criterion for conjugacy classes in the alternating group]]). (3) is the case of a conjugacy class that splits in <math>A_n</math> but each element still remains with its inverse. | Note that the permutations which satisfy condition (1) or (2) are precisely those whose conjugacy class is unsplit from the symmetric group, where every element is real. (see [[splitting criterion for conjugacy classes in the alternating group]]). (3) is the case of a conjugacy class that splits in <math>A_n</math> but each element still remains with its inverse. | ||

+ | |||

+ | ==Related facts== | ||

+ | |||

+ | ===Similar facts=== | ||

+ | |||

+ | * [[Splitting criterion for conjugacy classes in the alternating group]] | ||

+ | |||

+ | ===Applications=== | ||

+ | |||

+ | * [[Classification of ambivalent alternating groups]] | ||

+ | * [[Classification of alternating groups having a class-inverting automorphism]] |

## Latest revision as of 23:45, 29 August 2015

## Statement

Suppose is a natural number and is the Alternating group (?) on a set of size . An element is a Real element (?) if and only if the cycle decomposition of satisfies one of these three conditions:

- The cycle decomposition has a cycle of even length.
- The cycle decomposition has two cycles of equal odd length. Note that fixed points are counted as cycles of length 1, so this includes any permutation that has two or more fixed points.
- All the cycles have distinct odd lengths and is even. In other words, the number of s that are congruent to modulo is even.

Note that the permutations which satisfy condition (1) or (2) are precisely those whose conjugacy class is unsplit from the symmetric group, where every element is real. (see splitting criterion for conjugacy classes in the alternating group). (3) is the case of a conjugacy class that splits in but each element still remains with its inverse.