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Actions on a Set
Example
Failed to parse (unknown function "\lefttorightarrow"): {\displaystyle G \lefttorightarrow G}
by left (
) or right (
) multiplication.
now becomes
.
Observe that left and right multiplication commute:
.
Example
Vector space
and fields of scalars
,
, or
.
Then
is a (
,
, or
)-set, where
is the multiplicative group of nonzero elements of a field.
Example
Let
be the set of cosets of
formed from
.
Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle G \lefttorightarrow Y}
with
If Failed to parse (unknown function "\lefttorightarrow"): {\displaystyle G \lefttorightarrow X}
, then there is a bijection
such that
Example
Let
be the dihedral group
, where
is a rotation by
radians (in which case
is the identity element)
and
are mirroring on the vertical and horizontal axes, respectively
and
are mirroring along the NW-SE and NE-SW diagonals, respectively
Define set
of parts of the dihedral square:
through
are vertices
through
are sides
and
are vertical and horizontal axes, respectively
and
are the NW-SE and NE-SW diagonals, respectively
is the center point
through
are midpoints on each side
Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle D_4 \lefttorightarrow X}
.
We can fill out the operation table:
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is the orbit
is isotropy
Applications to Counting
- Isotropy group:

- Fixed Points:

Burnside Formula
Theorem 17.1. Let
be a finite group and
is a finite
-set. If
is a number of orbits in
under
, then
(⇒)
for each
.
is a disjoint union of
for each
, then
(⇒)