4 -periodic classes
We finally get to the black classes, which are -periodic. A “unit” of this periodicity looks like this:
The numbers on the class denote how many times it is divisible, relative to the coordinates in the diagram. For example, can be divided by twice to give , while doesn't even exist; only does.
The divisibilities of unlabelled classes are determined by and products (if divides, then so do and ). If a class is completely unlabelled, then its label should be interpreted to be .
Finally, the hollow classes are not actually in the diagram, but come from the C's. Their role is merely to indicate multiplications.
The differentials in follow from the differentials for via the Leibniz rule again. They look as follows:
The two “hooks” with lower left corner at and are periodic. The classes left (including the hollow ones) are killed by elements in .