|VW - CVM 1.1|
Recall that we use lattice coordinates, X and Y, when we describe lattice waves. These refer to the skewed coordinates along the directions of translation .
The most general function periodic with respect to any given lattice is a sum of terms of the form:
anm cos(nX) cos(mY)
+ bnmcos(nX) sin(mY) + cnmsin(nX) cos(mY) + dnmsin(nX) sin(mY)
Superimposing these will give a periodic function. How can additional symmetries be achieved?
We define several useful isometries that are important here: h is a horizontal half-translation, d a diagonal half-translation, v a vertical half-translation and R is a half-turn about the origin.
|The general lattice cell|
Each of these can be used as a negating symmetry to create a function invariant under p1 but with additional anti-symmetries. Some turn out to be algebraically equivalent. For instance, forcing h to be negating is algebraically the same as making d negating , so these are not traditionally thought of as being different.
Furthermore, we would not want to create functions with h, v, or d as actual symmetries, because these would then be symmetric in a finer lattice (one with a smaller cell). If the goal were to reduce the lattice, why not just start with a smaller one to begin with?
Thus there is only one new pattern type that can be formed from the group p1. For similar reasons, there is only one type that grows out of p2, the one where R is negative.
|type||G||E||Recipe for this type and remarks|
|p1||p1||p1||No additional symmetries;
use general parity, terms of all types
|p'b1||p1||p1||n is odd;
new cell is half of old
|p2'||p1||p2||only sin-cos and cos-sin terms are used;
|p2||p2||p2||only cos-cos and sin-sin terms appear|
|p'b2||p2||p2||p2 recipe, n + m is odd; negative half-turns;
new cell half of old