# Is $\mathbb Z _p^*=\{ 1, 2, 3, … , p-1 \}$ a cyclic group?

In undergraduate course, the two groups which are most frequently used may be $$\{ 0, 1, 2, ... , p-1\}$$ and $$\{ 1, 2, ... , p-1\}$$ where $p$ is a prime.

The first one is a group under addition and in addition it is a cyclic group whose generator is $p-1$. Also we can describe it by the solution set of $x^p=1$ in ${\bf C}$.

The latter is a group under multiplication. Fermat's theorem implies that $$a^{p-1} =1~~~ (\text{mod}\;\;p)$$ for $a\in \{ 1, ... , p-1\}$. But this is not sufficient for $\{ 1, ... , p-1\}$ to be cyclic.

My question is :

$$\{ 1, ... , p-1\}$$ is cyclic ?

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@Babak : Thank you for your distinguished editting. –  Hee Kwon Lee Jan 30 '13 at 9:44
For a proof, see math.stackexchange.com/questions/78416/…. –  lhf Jan 30 '13 at 10:24

Here is the proof that $G=\mathbb{F}_p ^*$ is cyclic (in fact any finite subgroup of the multiplicative group of a field is cyclic). One needs to know that a finite abelian group is the direct product of its Sylow subgroups: $$G \simeq G_{p_1} \times ...\times G_{p_n}$$ where $p_j$ is a prime and $G_{p_j}$ has order $p_j^{n_j}$. So you can assume that the group's order is a power of a prime, say $G$ has order $q^n$. But then, if $G$ does not contain an element of order $q^n$, the order of every element divides $q^{n-1}$, that is, for all $a\in G$, $$a^{q^{n-1}}-1=0$$ in the field. But this cannot be true because the polynomial of order $q^{n-1}$ would have $q^n$ roots.

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The group $\{1,2,\ldots,p-1\}$ under multiplication is indeed cyclic. It can be shown that there exists a primitive root $\pmod p$ for every prime $p$. This will now generate the multiplicative group $\{1,2,\ldots,p-1\}$.

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It is a general theorem that every finite subgroup of the multiplicative group of a field is cyclic. That is, if $F$ is any field, the set $F^*$ of all non-zero elements in $F$ is always a group under the multiplication in the field. Any finite subgroup of $F^*$ is cyclic. Thus the group $\mathbb Z_p^*$ that you describe is cyclic, since $\mathbb Z_p$ is a field.

There are several proofs for this result, one of them uses a characterization of finite cyclic groups that can be applied directly to $Z_p^*$. The characterization of finite cyclic groups is that if $G$ is a finite group of order $n$ then $G$ is cyclic if, and only if, for very $d$ that divides $n$ the group $G$ has at most one subgroup of order $d$. The proof is not very hard and is classical. It should be noted that the proof does not point to any generator, but only proves the existence of a generator. Consequently, using this theorem to prove that $\mathbb Z_p^*$ is cyclic does not produce a generator (known as a primitive element). There are no trivial ways to produce such primitive elements.

A somewhat similar situation is the result from Galois theory that every finite separable extension is generated by one element (a primitive element again). The general proof is again non-constructive and finding primitive elements for any given field extension can be very hard.

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Consider the field $\mathbb{F}_{p}$ - the field with $p$ elements.

The group $\{1,...p\}$ is $\mathbb{F}_{p}^{*}$, that is all the invertible elements of $\mathbb{F}_{p}$.

This is a cyclic group as it is a finite subgroup of a field (with respect to multiplication).

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It is cyclic. A generator $a \in \mathbb F_p^*$ would satisfy $|a| = p - 1$ where $|a|$ is the order of $a$. We can find the generator as follows. Factor $p - 1$ as $p - 1 = q_1^{a_1}q_2^{a_2}\ldots q_n^{a_n}$ where $q_i$ are distinct primes, and $a_i$ are all non-zero.
1. We will try to find elements of $\mathbb F_p^*$ that have orders $q_i^{a_i}$. Since $x^{p-1} - 1 = 0$ has $p - 1$ roots (by Fermat's theorem) and $x^{p-1} - 1$ factors, for any $i$, as \begin{align*} x^{p-1} - 1 & = \left(x^{q_i^{a_i}} - 1\right)\left(1 + x^{q_i^{a_i}} + \ldots + \left(x^{q_i^{a_i}}\right)^{(p-1)q_i^{-a_i} - 1}\right), \end{align*} we know that $x^{q_i^{a_i}} - 1$ has exactly $q_i^{a_i}$ roots by counting degrees. A similar argument can be made to conclude that $x^{q_i^{a_i - 1}} - 1$ has exactly $q_i^{a_i - 1}$ roots. Therefore, there exists some $g_i \in \mathbb F_p^*$ such that $g^{q_i^{a_i}} = 1$ and $g^{q_i^{a_i - 1}} \ne 1$. We see that $|g| \mid q_i^{a_i}$ but $|g| \nmid q_i^{a_i - 1}$, so $|g| = q_i^{a_i}$.
2. Let $g = g_1g_2\ldots g_n$. Obviously $|g| \mid p - 1$, i.e., $|g| = q_1^{b_1}q_2^{b_2}\ldots q_n^{b_n}$ with $b_n \le a_n$. Write this as $$\prod_{i} g_i^{q_1^{b_1}q_2^{b_2}\ldots q_n^{b_n}} = 1.$$ Fix $j \in \{1, 2, \ldots, n\}$. Raise the above equation to the $q_k^{a_k - b_k}$-th power for all $k \ne j$ successively: \begin{align*} \prod_{i} g_i^{(p-1)q_j^{b_j-a_j}} & = 1. \end{align*} For $i \ne j$, the factor $g_i^{(p-1)q_j^{b_j-a_j}} = 1$ because $|g_i| = q_i^{a_i} \mid (p-1)q_j^{b_j - a_j}$. So the only non-identity factor is the term $i = j$: $$g_j^{(p-1)q_j^{b_j-a_j}} = 1.$$ This means $|g_j| \mid (p-1)q_j^{b_j - a_j} = (p-1)q_j^{-a_j}q_j^{b_j}$, but $|g_j| = q_j^{a_j}$ and $q_j \nmid (p-1)q_j^{-a_j}$, we must have $b_j = a_j$.