Symmetry
The Life transition rule, like that of any isotropic cellular automaton, is invariant under reflections and rotations. That is, the change in state of a cell remains the same if its neighbourhood is rotated or reflected. This implies there are symmetries which if present in a pattern are present in all its successors. Note that the converse is not true: a pattern need not have the full symmetry of one of its successor states.
Rotational symmetries
Rotational symmetries include the following (note that "C" refers to the cyclic groups):
C1
C1: Symmetric under 360° rotation. This is essentially no symmetry at all.
C2
C2: Symmetric under 180° rotation. There are three possibilities:
- C2_1: Rotation around the center of a cell. The bounding rectangle of a C2_1 pattern is odd by odd.
- C2_2: Rotation around the midpoint of a side of a cell. The bounding rectangle is even by odd.
- C2_4: Rotation around a corner of a cell. The bounding rectangle is even by even.
C4
C4: Symmetric under 90° rotation. There are two possibilities:
- C4_1: Rotation around the center of a cell. The bounding rectangle is odd by odd.
- C4_4: Rotation around a corner of a cell. The bounding rectangle is even by even.
Reflectional symmetries
Reflectional symmetries include the following (note that "D" refers to the dihedral groups):
D2
D2: Symmetric under reflection through a line. There are two possibilities:
- D2_+ The line is orthogonal. There are two sub-possibilities:
- D2_+1 The line bisects a row of cells. The bounding rectangle is odd by any.
- D2_+2 The line lies between two rows of cells. The bounding rectangle is even by any.
- D2_x The line is diagonal.
D4
D4: Symmetric under both reflection and 180° rotation. The reflection symmetry will be with respect to two lines. There are two possibilities:
- D4_+: The lines are orthogonal. There are three sub-possibilities:
- D4_+1: Rotation around the center of a cell. The bounding rectangle is odd by odd.
- D4_+2: Rotation around the midpoint of a side of a cell. The bounding rectangle is even by odd.
- D4_+4: Rotation around a corner of a cell. The bounding rectangle is even by even.
- D4_x The lines are diagonal. There are two sub-possibilities:
- D4_x1: Rotation around the center of a cell. The bounding rectangle is odd by odd.
- D4_x4: Rotation around a corner of a cell. The bounding rectangle is even by even.
D8
D8: Symmetric under both reflection and 90° rotation. The reflection symmetry will be with respect to horizontal, vertical, and diagonal lines. There are two possibilities:
- D8_1: Rotation around the center of a cell. The bounding rectangle is odd by odd.
- D8_2: Rotation around a edge of a cell. The bounding rectangle is even by odd. This symmetry is not preserved by Life (reverting to D4_+2), but is with most bilaterally symmetric rules.
- D8_4: Rotation around a corner of a cell. The bounding rectangle is even by even.
Skew symmetries
If a pattern exhibits symmetry only after its constituent congruent pieces are offset by certain amounts in one or both orthogonal directions, the pattern is said to exhibit skew symmetry.
Gutter symmetries
Gutter symmetries are distinguished from non-gutter symmetries by the existence of an empty lane of cells – the "gutter" – separating the congruent pieces making up overall pattern.
A pattern that exhibits gutter symmetry only after its pieces are skewed in the above sense is said to exhibit skew-gutter symmetry.
References
- Help with symmetries (discussion thread) at the ConwayLife.com forums