How do you determine the accumulation point of a set?

For example,

What is the accumulation point of $\displaystyle z_{n}$ = $\displaystyle i^n$ where n $\displaystyle \in$ $\displaystyle Z^+$

And what is the accumulation point of the following?

(a) $\displaystyle z_{n}$ = $\displaystyle i^n$/n where n $\displaystyle \in$ $\displaystyle Z^+$

(b) 0 $\displaystyle \leq$ arg(z) < $\displaystyle \pi$/2 and z not equal to 0

(c) $\displaystyle z_{n}$ = $\displaystyle (-1)^n$(1+i) (n-1)/n where n $\displaystyle \in$ $\displaystyle Z^+$

Any help? Thanks!