Yes, a set seems appropriate (unless the tasks have to be performed in a particular order, then a suitable tuple (which includes an ordering for the tasks) would make more sense).
You could represent the task set of worker $k$ as follows:
$${\rm Tasks}(k) = \\{m\mid 1 \le m \le n \text{ and } m \equiv k \pmod p\\}$$
where $m \equiv k \pmod p$ simply denotes that $m$ and $k$ have the same remainder modulo $p$.
In an ordered tuple rendering, you'd probably end up with something like:
$${\rm Tasks}(k) = (k, k+p, \ldots,k+lp)$$
where $l = \left\lfloor\frac{n-k}p\right\rfloor$ is used to determine the last task that worker $k$ has to perform.