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The term order should work well.Is there a term for the least k such that a^k = 1 in some ring R (if it exists, specifically in a finite ring)?

Let ##n## be the number of invertible elements in the ring, then ##n## is an upper bound.Is there a way to get an upper bound in general?

Well, you know that the invertible elements in ##\mathbb{F}_p## form a cyclic group of order ##p-1##. So finding the orders of the elements in equivalent to finding the orders of elements in ##\mathbb{Z}_{p-1}##, which is easy. So you know exactly which orders show up and which don't. The hard part is finding an explicit isomorphism between ##\mathbb{Z}_{p-1}## and the invertible elements in ##\mathbb{F}_p## (or equivalently: finding an element in ##\mathbb{F}_p## with order ##p-1##).I know that it's hard to get even in say F_p

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