# Cyclotomic field

Template:No footnotes
In number theory, a **cyclotomic field** is a number field obtained by adjoining a complex primitive root of unity to **Q**, the field of rational numbers. The Template:Mvar-th cyclotomic field **Q**(ζ_{n}) (where *n* > 2) is obtained by adjoining a primitive Template:Mvar-th root of unity ζ_{n} to the rational numbers.

The cyclotomic fields played a crucial role in the development of modern algebra and number theory because of their relation with Fermat's last theorem. It was in the process of his deep investigations of the arithmetic of these fields (for prime Template:Mvar) – and more precisely, because of the failure of unique factorization in their rings of integers – that Ernst Kummer first introduced the concept of an ideal number and proved his celebrated congruences.

## Properties

A cyclotomic field is the splitting field of the cyclotomic polynomial

and therefore it is a Galois extension of the field of rational numbers. The degree of the extension

- [
**Q**(ζ_{n}):**Q**]

is given by *φ*(*n*) where Template:Mvar is Euler's phi function. A complete set of Galois conjugates is given by { (ζ_{n})^{a} } , where Template:Mvar runs over the set of invertible residues modulo Template:Mvar (so that Template:Mvar is relative prime to Template:Mvar). The Galois group is naturally isomorphic to the multiplicative group

- (
**Z**/*n***Z**)^{×}

of invertible residues modulo Template:Mvar, and it acts on the primitive Template:Mvarth roots of unity by the formula

*b*: (ζ_{n})^{a}→ (ζ_{n})^{a b}.

## Relation with regular polygons

Gauss made early inroads in the theory of cyclotomic fields, in connection with the geometrical problem of constructing a [[regular polygon|regular Template:Mvar-gon]] with a compass and straightedge. His surprising result that had escaped his predecessors was that a regular heptadecagon (with 17 sides) could be so constructed. More generally, if Template:Mvar is a prime number, then a regular Template:Mvar-gon can be constructed if and only if Template:Mvar is a Fermat prime; in other words if is a power of 2.

For *n* = 3 and *n* = 6 primitive roots of unity admit a simple expression via square root of three, namely:

- ζ
_{3}= {{ safesubst:#invoke:Unsubst||$B=Template:Sqrt*i*− 1/2}}, ζ_{6}= {{ safesubst:#invoke:Unsubst||$B=Template:Sqrt*i*+ 1/2}}

Hence, both corresponding cyclotomic fields are identical to the quadratic field **Q**(Template:Sqrt). In the case of ζ_{4} = *i* = Template:Sqrt the identity of **Q**(ζ_{4}) to a quadratic field is even more obvious. This is not the case for *n* = 5 though, because expressing roots of unity requires square roots *of* quadratic integers, that means that roots belong to a second iteration of quadratic extension. The geometric problem for a general Template:Mvar can be reduced to the following question in Galois theory: can the Template:Mvarth cyclotomic field be built as a sequence of quadratic extensions?

## Relation with Fermat's Last Theorem

A natural approach to proving Fermat's Last Theorem is to factor the binomial *x ^{n}* +

*y*, where Template:Mvar is an odd prime, appearing in one side of Fermat's equation

^{n}*x*+^{n}*y*=^{n}*z*^{n}

as follows:

*x*+^{n}*y*= (^{n}*x*+*y*) (*x*+ ζ*y*) … (*x*+ ζ^{n − 1}*y*).

Here Template:Mvar and Template:Mvar are ordinary integers, whereas the factors are algebraic integers in the cyclotomic field **Q**(ζ_{n}). If unique factorization of algebraic integers were true, then it could have been used to rule out the existence of nontrivial solutions to Fermat's equation.

Several attempts to tackle Fermat's Last Theorem proceeded along these lines, and both Fermat's proof for *n* = 4 and Euler's proof for *n* = 3 can be recast in these terms. Unfortunately, the unique factorization fails in general – for example, for *n* = 23 – but Kummer found a way around this difficulty. He introduced a replacement for the prime numbers in the cyclotomic field **Q**(ζ_{p}), expressed the failure of unique factorization quantitatively via the class number *h _{p}* and proved that if

*h*is not divisible by Template:Mvar (such numbers Template:Mvar are called

_{p}*regular primes*) then Fermat's theorem is true for the exponent

*n*=

*p*. Furthermore, he gave a criterion to determine which primes are regular and using it, established Fermat's theorem for all prime exponents Template:Mvar less than 100, with the exception of the

*irregular primes*37, 59, and 67. Kummer's work on the congruences for the class numbers of cyclotomic fields was generalized in the twentieth century by Iwasawa in Iwasawa theory and by Kubota and Leopoldt in their theory of p-adic zeta functions.

### List of Class Numbers to Cyclotomic Field

(sequence A061653 in OEIS), or Template:Oeis (for prime *n*)

n | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13 | 14 | 15 | 16 | 17 | 18 | 19 | 20 |

Class number | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 |

n | 21 | 22 | 23 | 24 | 25 | 26 | 27 | 28 | 29 | 30 | 31 | 32 | 33 | 34 | 35 | 36 | 37 | 38 | 39 | 40 |

Class number | 1 | 1 | 3 | 1 | 1 | 1 | 1 | 1 | 8 | 1 | 9 | 1 | 1 | 1 | 1 | 1 | 37 | 1 | 2 | 1 |

n | 41 | 42 | 43 | 44 | 45 | 46 | 47 | 48 | 49 | 50 | 51 | 52 | 53 | 54 | 55 | 56 | 57 | 58 | 59 | 60 |

Class number | 121 | 1 | 211 | 1 | 1 | 3 | 695 | 1 | 43 | 1 | 5 | 3 | 4889 | 1 | 10 | 2 | 9 | 8 | 41241 | 1 |

n | 61 | 62 | 63 | 64 | 65 | 66 | 67 | 68 | 69 | 70 | 71 | 72 | 73 | 74 | 75 | 76 | 77 | 78 | 79 | 80 |

Class number | 76301 | 9 | 7 | 17 | 64 | 1 | 853513 | 8 | 69 | 1 | 3882809 | 3 | 11957417 | 37 | 11 | 19 | 1280 | 2 | 100146415 | 5 |

n | 81 | 82 | 83 | 84 | 85 | 86 | 87 | 88 | 89 | 90 | 91 | 92 | 93 | 94 | 95 | 96 | 97 | 98 | 99 | 100 |

Class number | 2593 | 121 | 838216959 | 1 | 6205 | 211 | 1536 | 55 | 13379363737 | 1 | 53872 | 201 | 6795 | 695 | 107692 | 9 | 411322824001 | 43 | 2883 | 55 |

n | 101 | 102 | 103 | 104 | 105 | 106 | 107 | 108 | 109 | 110 | 111 | 112 | 113 | 114 | 115 | 116 | 117 | 118 | 119 | 120 |

Class number | 3547404378125 | 5 | 9069094643165 | 351 | 13 | 4889 | 63434933542623 | 19 | 161784800122409 | 10 | 480852 | 468 | 1612072001362952 | 9 | 44697909 | 10752 | 132678 | 41241 | 1238459625 | 4 |

n | 121 | 122 | 123 | 124 | 125 | 126 | 127 | 128 | 129 | 130 | 131 | 132 | 133 | 134 | 135 | 136 | 137 | 138 | 139 | 140 |

Class number | 12188792628211 | 76301 | 8425472 | 75456 | 57708445601 | 7 | 2604529186263992195 | 359057 | 37821539 | 64 | 28496379729272136525 | 11 | 157577452812 | 853513 | 75961 | 111744 | 646901570175200968153 | 69 | 1753848916484925681747 | 39 |

n | 141 | 142 | 143 | 144 | 145 | 146 | 147 | 148 | 149 | 150 | 151 | 152 | 153 | 154 | 155 | 156 | 157 | 158 | 159 | 160 |

Class number | 1257700495 | 3882809 | 36027143124175 | 507 | 1467250393088 | 11957417 | 5874617 | 4827501 | 687887859687174720123201 | 11 | 2333546653547742584439257 | 1666737 | 2416282880 | 1280 | 84473643916800 | 156 | 56234327700401832767069245 | 100146415 | 223233182255 | 31365 |

## See also

## References

- Bryan Birch, "Cyclotomic fields and Kummer extensions", in J.W.S. Cassels and A. Frohlich (edd),
*Algebraic number theory*, Academic Press, 1973. Chap.III, pp. 45–93. - Daniel A. Marcus,
*Number Fields*, third edition, Springer-Verlag, 1977 - Lawrence C. Washington,
*Introduction to Cyclotomic Fields*, Graduate Texts in Mathematics, 83. Springer-Verlag, New York, 1982. ISBN 0-387-90622-3 - Serge Lang,
*Cyclotomic Fields I and II*, Combined second edition. With an appendix by Karl Rubin. Graduate Texts in Mathematics, 121. Springer-Verlag, New York, 1990. ISBN 0-387-96671-4

## Further reading

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- Weisstein, Eric W., "Cyclotomic Field",
*MathWorld*. - {{#invoke:citation/CS1|citation

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