Math test
Inline math:
\(e^{i\pi} + 1 = 0\)
Display math:
$$
q = p^n
$$
A slightly more interesting one:
$$
\mathbf{F}_{p^n}^{\times}
\cong C_{p^n - 1}.
$$
And:
$$
\int_0^\infty e^{-x^2}\,dx
=
\frac{\sqrt{\pi}}{2}.
$$
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Finite Fields up to Order 128
Let \(\zeta_n\) denote a primitive \(n\)th root of unity.
For every finite field \(\mathbf F_q\),
$$
\mathbf F_q^\times \cong C_{q-1}=\mu_{q-1}.
$$
In particular, if \(q=p^d\), then
$$
\mathbf F_q=\mathbf F_p(\zeta_{q-1}),
$$
although a root of unity of much smaller order often already generates the extension.
| Field | Units | Cyclotomic realizations | Why it is beautiful |
|---|---|---|---|
| \(\mathbf F_2\) | \(\mathbf F_2^\times\cong C_1\) | \(\mathbf F_2=\mathbf F_2(\zeta_1)\) | The atom. Its entire multiplicative group is trivial and \(-1=1\). Characteristic \(2\) begins its campaign against ordinary intuition. |
| \(\mathbf F_3\) | \(\mathbf F_3^\times\cong C_2\) | \(\mathbf F_3=\mathbf F_3(\zeta_2)\) | The smallest field where \(1\neq-1\). The familiar pair \(\{\pm1\}\) finally exists. |
| \(\mathbf F_4\) | \(\mathbf F_4^\times\cong C_3\) | \(\mathbf F_4=\mathbf F_2(\zeta_3)\) | The first genuine finite-field extension. Its three nonzero elements are exactly the cube roots of unity. Tiny perfection. |
| \(\mathbf F_5\) | \(\mathbf F_5^\times\cong C_4\) | \(\mathbf F_5=\mathbf F_5(\zeta_4)\) | Already contains a primitive fourth root of unity: \(2^2=-1\). An \(i\)-like element appears inside the prime field itself. |
| \(\mathbf F_7\) | \(\mathbf F_7^\times\cong C_6\) | \(\mathbf F_7=\mathbf F_7(\zeta_6)\) | Contains primitive \(3\)rd and \(6\)th roots of unity. The factorization \(6=2\cdot3\) gives a surprisingly rich tiny multiplicative world. |
| \(\mathbf F_8\) | \(\mathbf F_8^\times\cong C_7\) | \(\mathbf F_8=\mathbf F_2(\zeta_7)\) | The first cubic extension. Since \(7\) is prime, every nonzero element except \(1\) is a primitive \(7\)th root of unity. |
| \(\mathbf F_9\) | \(\mathbf F_9^\times\cong C_8\) | \(\mathbf F_9=\mathbf F_3(\zeta_4)=\mathbf F_3(\zeta_8)\) | The finite-field analogue of adjoining \(i\): \(\mathbf F_9=\mathbf F_3(i)\). Its entire unit group is \(\mu_8\). |
| \(\mathbf F_{11}\) | \(\mathbf F_{11}^\times\cong C_{10}\) | \(\mathbf F_{11}=\mathbf F_{11}(\zeta_{10})\) | Primitive fifth roots of unity already live in the prime field because \(5\mid10\). |
| \(\mathbf F_{13}\) | \(\mathbf F_{13}^\times\cong C_{12}\) | \(\mathbf F_{13}=\mathbf F_{13}(\zeta_{12})\) | Contains primitive roots of orders \(3,4,6,\) and \(12\). A very rich little cyclotomic playground. |
| \(\mathbf F_{16}\) | \(\mathbf F_{16}^\times\cong C_{15}\) | \(\mathbf F_{16}=\mathbf F_2(\zeta_5)=\mathbf F_2(\zeta_{15})\) | The famous counterintuitive step: \(\zeta_3\) gives \(\mathbf F_4\), \(\zeta_7\) gives \(\mathbf F_8\), but the smaller \(\zeta_5\) needs degree \(4\). It also contains \(\mathbf F_4\). |
| \(\mathbf F_{17}\) | \(\mathbf F_{17}^\times\cong C_{16}\) | \(\mathbf F_{17}=\mathbf F_{17}(\zeta_{16})\) | Pure \(2\)-power cyclotomy: \(\mathbf F_{17}^\times\) has order \(2^4\). Fermat-prime elegance appears here. |
| \(\mathbf F_{19}\) | \(\mathbf F_{19}^\times\cong C_{18}\) | \(\mathbf F_{19}=\mathbf F_{19}(\zeta_{18})\) | Since \(18=2\cdot3^2\), primitive \(9\)th and \(18\)th roots of unity occur naturally. |
| \(\mathbf F_{23}\) | \(\mathbf F_{23}^\times\cong C_{22}\) | \(\mathbf F_{23}=\mathbf F_{23}(\zeta_{22})\) | Its multiplicative group contains the large prime-order subgroup \(\mu_{11}\). Clean and almost prime-order. |
| \(\mathbf F_{25}\) | \(\mathbf F_{25}^\times\cong C_{24}\) | \(\mathbf F_{25}=\mathbf F_5(\zeta_3)=\mathbf F_5(\zeta_{24})\) | A lovely surprise: merely adjoining a primitive cube root of unity to \(\mathbf F_5\) produces the quadratic extension \(\mathbf F_{25}\). |
| \(\mathbf F_{27}\) | \(\mathbf F_{27}^\times\cong C_{26}\) | \(\mathbf F_{27}=\mathbf F_3(\zeta_{13})=\mathbf F_3(\zeta_{26})\) | A primitive \(13\)th root creates a cubic extension because \(\operatorname{ord}_{13}(3)=3\). There are no intermediate fields. |
| \(\mathbf F_{29}\) | \(\mathbf F_{29}^\times\cong C_{28}\) | \(\mathbf F_{29}=\mathbf F_{29}(\zeta_{28})\) | Since \(28=2^2\cdot7\), fourth and seventh roots of unity coexist naturally. |
| \(\mathbf F_{31}\) | \(\mathbf F_{31}^\times\cong C_{30}\) | \(\mathbf F_{31}=\mathbf F_{31}(\zeta_{30})\) | The factorization \(30=2\cdot3\cdot5\) gives elements of orders \(2,3,5,6,10,15,\) and \(30\). |
| \(\mathbf F_{32}\) | \(\mathbf F_{32}^\times\cong C_{31}\) | \(\mathbf F_{32}=\mathbf F_2(\zeta_{31})\) | An absolute jewel. Since \(31\) is prime, every element of \(\mathbf F_{32}\setminus\{0,1\}\) is a primitive \(31\)st root of unity. |
| \(\mathbf F_{37}\) | \(\mathbf F_{37}^\times\cong C_{36}\) | \(\mathbf F_{37}=\mathbf F_{37}(\zeta_{36})\) | The unit group has order \(36=2^2\cdot3^2\), giving a rich mixture of \(2\)-power and \(3\)-power cyclotomy. |
| \(\mathbf F_{41}\) | \(\mathbf F_{41}^\times\cong C_{40}\) | \(\mathbf F_{41}=\mathbf F_{41}(\zeta_{40})\) | Since \(40=2^3\cdot5\), eighth roots and fifth roots of unity coexist inside one prime field. |
| \(\mathbf F_{43}\) | \(\mathbf F_{43}^\times\cong C_{42}\) | \(\mathbf F_{43}=\mathbf F_{43}(\zeta_{42})\) | \(42=2\cdot3\cdot7\): quadratic, cubic, and seventh-root cyclotomy all coexist. |
| \(\mathbf F_{47}\) | \(\mathbf F_{47}^\times\cong C_{46}\) | \(\mathbf F_{47}=\mathbf F_{47}(\zeta_{46})\) | Its multiplicative structure is dominated by the large prime-order subgroup \(\mu_{23}\). |
| \(\mathbf F_{49}\) | \(\mathbf F_{49}^\times\cong C_{48}\) | \(\mathbf F_{49}=\mathbf F_7(\zeta_4)=\mathbf F_7(\zeta_{48})\) | Beautifully simple: adjoining \(i\) to \(\mathbf F_7\) produces \(\mathbf F_{49}\). Its unit group has order \(48=2^4\cdot3\). |
| \(\mathbf F_{53}\) | \(\mathbf F_{53}^\times\cong C_{52}\) | \(\mathbf F_{53}=\mathbf F_{53}(\zeta_{52})\) | Since \(52=2^2\cdot13\), it is a natural home for primitive \(13\)th roots of unity. |
| \(\mathbf F_{59}\) | \(\mathbf F_{59}^\times\cong C_{58}\) | \(\mathbf F_{59}=\mathbf F_{59}(\zeta_{58})\) | Its cyclotomic structure is dominated by the large prime \(29\). |
| \(\mathbf F_{61}\) | \(\mathbf F_{61}^\times\cong C_{60}\) | \(\mathbf F_{61}=\mathbf F_{61}(\zeta_{60})\) | One of the richest prime-field playgrounds in this range: \(60=2^2\cdot3\cdot5\) has many divisors and therefore many root orders. |
| \(\mathbf F_{64}\) | \(\mathbf F_{64}^\times\cong C_{63}\) | \(\mathbf F_{64}=\mathbf F_2(\zeta_9)=\mathbf F_2(\zeta_{21})=\mathbf F_2(\zeta_{63})\) | A primitive \(9\)th root already generates the sextic extension. It contains both the \(\zeta_3\) world \(\mathbf F_4\) and the \(\zeta_7\) world \(\mathbf F_8\). |
| \(\mathbf F_{67}\) | \(\mathbf F_{67}^\times\cong C_{66}\) | \(\mathbf F_{67}=\mathbf F_{67}(\zeta_{66})\) | Since \(66=2\cdot3\cdot11\), it simultaneously contains primitive \(3\)rd, \(6\)th, \(11\)th, \(22\)nd, \(33\)rd, and \(66\)th roots. |
| \(\mathbf F_{71}\) | \(\mathbf F_{71}^\times\cong C_{70}\) | \(\mathbf F_{71}=\mathbf F_{71}(\zeta_{70})\) | \(70=2\cdot5\cdot7\), so fifth- and seventh-root cyclotomy meet naturally. |
| \(\mathbf F_{73}\) | \(\mathbf F_{73}^\times\cong C_{72}\) | \(\mathbf F_{73}=\mathbf F_{73}(\zeta_{72})\) | \(72=2^3\cdot3^2\) gives unusually rich simultaneous \(2\)-power and \(3\)-power root-of-unity structure. |
| \(\mathbf F_{79}\) | \(\mathbf F_{79}^\times\cong C_{78}\) | \(\mathbf F_{79}=\mathbf F_{79}(\zeta_{78})\) | Since \(78=2\cdot3\cdot13\), primitive \(13\)th roots coexist with cubic structure. |
| \(\mathbf F_{81}\) | \(\mathbf F_{81}^\times\cong C_{80}\) | \(\mathbf F_{81}=\mathbf F_3(\zeta_5)=\mathbf F_3(\zeta_{16})=\mathbf F_3(\zeta_{80})\) | A primitive fifth root already generates the quartic extension because \(\operatorname{ord}_5(3)=4\). It also contains \(\mathbf F_9\) as a proper subfield. |
| \(\mathbf F_{83}\) | \(\mathbf F_{83}^\times\cong C_{82}\) | \(\mathbf F_{83}=\mathbf F_{83}(\zeta_{82})\) | Its unit group has order \(82=2\cdot41\), dominated by the large prime cyclotomic subgroup \(\mu_{41}\). |
| \(\mathbf F_{89}\) | \(\mathbf F_{89}^\times\cong C_{88}\) | \(\mathbf F_{89}=\mathbf F_{89}(\zeta_{88})\) | \(88=2^3\cdot11\) combines substantial \(2\)-power structure with primitive \(11\)th roots. |
| \(\mathbf F_{97}\) | \(\mathbf F_{97}^\times\cong C_{96}\) | \(\mathbf F_{97}=\mathbf F_{97}(\zeta_{96})\) | \(96=2^5\cdot3\): a remarkable amount of \(2\)-power cyclotomy occurs inside a prime field of fewer than \(100\) elements. |
| \(\mathbf F_{101}\) | \(\mathbf F_{101}^\times\cong C_{100}\) | \(\mathbf F_{101}=\mathbf F_{101}(\zeta_{100})\) | The particularly tidy factorization \(100=2^2\cdot5^2\) gives rich fifth-power cyclotomic structure. |
| \(\mathbf F_{103}\) | \(\mathbf F_{103}^\times\cong C_{102}\) | \(\mathbf F_{103}=\mathbf F_{103}(\zeta_{102})\) | Since \(102=2\cdot3\cdot17\), primitive \(17\)th roots sit alongside cubic and sixth-root structure. |
| \(\mathbf F_{107}\) | \(\mathbf F_{107}^\times\cong C_{106}\) | \(\mathbf F_{107}=\mathbf F_{107}(\zeta_{106})\) | Its multiplicative group has the simple shape \(C_{2\cdot53}\), with a large prime-order subgroup \(\mu_{53}\). |
| \(\mathbf F_{109}\) | \(\mathbf F_{109}^\times\cong C_{108}\) | \(\mathbf F_{109}=\mathbf F_{109}(\zeta_{108})\) | \(108=2^2\cdot3^3\), so this field is especially rich in \(3\)-power roots of unity, including primitive \(27\)th roots. |
| \(\mathbf F_{113}\) | \(\mathbf F_{113}^\times\cong C_{112}\) | \(\mathbf F_{113}=\mathbf F_{113}(\zeta_{112})\) | \(112=2^4\cdot7\) combines deep \(2\)-power cyclotomy with primitive seventh roots. |
| \(\mathbf F_{121}\) | \(\mathbf F_{121}^\times\cong C_{120}\) | \(\mathbf F_{121}=\mathbf F_{11}(\zeta_3)=\mathbf F_{11}(\zeta_{120})\) | A primitive cube root already generates the quadratic extension of \(\mathbf F_{11}\). Meanwhile \(120=2^3\cdot3\cdot5\) makes its unit group spectacularly rich. |
| \(\mathbf F_{125}\) | \(\mathbf F_{125}^\times\cong C_{124}\) | \(\mathbf F_{125}=\mathbf F_5(\zeta_{31})=\mathbf F_5(\zeta_{124})\) | A primitive \(31\)st root creates a cubic extension because \(\operatorname{ord}_{31}(5)=3\). Since the degree is prime, there are no intermediate fields. |
| \(\mathbf F_{127}\) | \(\mathbf F_{127}^\times\cong C_{126}\) | \(\mathbf F_{127}=\mathbf F_{127}(\zeta_{126})\) | \(126=2\cdot3^2\cdot7\) gives a rich mixture of cubic, ninth-root, and seventh-root behavior. Also, \(127\) itself is a Mersenne prime. |
| \(\mathbf F_{128}\) | \(\mathbf F_{128}^\times\cong C_{127}\) | \(\mathbf F_{128}=\mathbf F_2(\zeta_{127})\) | Peak tiny binary-field behavior. Since \(127\) is prime, every element of \(\mathbf F_{128}\setminus\{0,1\}\) is a primitive \(127\)th root of unity. The extension degree \(7\) is prime, so there are no intermediate fields. |