IOTA Whitepaper: What is z in Section 4.1?

Question

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.

Answer 1

Variable z is just the index over which the sum in equation (13) iterates. This is similar to when you have

 \sum_{i=1}^n ...

where i is the standard index (so no need to "define" i separately).

Now, notation 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 z.