Everything in the equation is defined except z. Is it supposed to be another parameter that is chosen like alpha? I want to actually use some reasonable sample variables and test this.
z is just the index over which the sum in equation (13) iterates. This is similar to when you have
i is the standard index (so no need to "define"
z:z⇝x simply means
All z such that z approves x or
All z such that z⇝x. So we sum over all transactions
z that directly approve our transaction
x. Note that
y is one of these