Arithmetic sequences have a common difference between consecutive terms.
Geometric sequences have a common ratio between consecutive terms.
Let's compute the differences and ratios between consecutive terms:
Differences:

Ratios:

So, as you can see, the differences between consecutive terms are constant, whereas ratios vary.
So, this is an arithmetic sequence.
ANSWER
The restrictions are

EXPLANATION
We were given the rational function,

The function is defined for all values of a, except

This has become a quadratic trinomial, so we need to split the middle term.
We do that by multiplying the coefficient of
which is 5 by the constant term which is 3. This gives us 15.
The factors of 15 that adds up to 16 are 1 and 15.
We use these factors to split the middle term.

We now factor to get,

We factor further to get,

This implies that,

This gives

These are the restrictions.