# Fundamental unit (number theory)

In algebraic number theory, a fundamental unit is a generator (modulo the roots of unity) for the unit group of the ring of integers of a number field, when that group has rank 1 (i.e. when the unit group modulo its torsion subgroup is infinite cyclic). Dirichlet's unit theorem shows that the unit group has rank 1 exactly when the number field is a real quadratic field, a complex cubic field, or a totally imaginary quartic field. When the unit group has rank ≥ 1, a basis of it modulo its torsion is called a fundamental system of units.[1] Some authors use the term fundamental unit to mean any element of a fundamental system of units, not restricting to the case of rank 1 (e.g. Neukirch 1999, p. 42).

For the real quadratic field ${\displaystyle K=\mathbf {Q} ({\sqrt {d}})}$ (with d square-free), the fundamental unit ε is commonly normalized so that ε > 1 (as a real number). Then it is uniquely characterized as the minimal unit among those that are greater than 1. If Δ denotes the discriminant of K, then the fundamental unit is

${\displaystyle \varepsilon ={\frac {a+b{\sqrt {\Delta }}}{2}}}$

where (a, b) is the smallest solution to[2]

${\displaystyle x^{2}-\Delta y^{2}=\pm 4}$

in positive integers. This equation is basically Pell's equation or the negative Pell equation and its solutions can be obtained similarly using the continued fraction expansion of ${\displaystyle {\sqrt {\Delta }}}$.

Whether or not x2  Δy2 = −4 has a solution determines whether or not the class group of K is the same as its narrow class group, or equivalently, whether or not there is a unit of norm −1 in K. This equation is known to have a solution if, and only if, the period of the continued fraction expansion of ${\displaystyle {\sqrt {\Delta }}}$ is odd. A simpler relation can be obtained using congruences: if Δ is divisible by a prime that is congruent to 3 modulo 4, then K does not have a unit of norm −1. However, the converse does not hold as shown by the example d = 34.[3] In the early 1990s, Peter Stevenhagen proposed a probabilistic model that led him to a conjecture on how often the converse fails. Specifically, if D(X) is the number of real quadratic fields whose discriminant Δ < X is not divisible by a prime congruent to 3 modulo 4 and D(X) is those who have a unit of norm −1, then[4]

${\displaystyle \lim _{X\rightarrow \infty }{\frac {D^{-}(X)}{D(X)}}=1-\prod _{j\geq 1{\text{ odd}}}\left(1-2^{-j}\right).}$

In other words, the converse fails about 42% of the time. As of March 2012, a recent result towards this conjecture was provided by Étienne Fouvry and Jürgen Klüners[5] who show that the converse fails between 33% and 59% of the time.

## Cubic fields

If K is a complex cubic field then it has a unique real embedding and the fundamental unit ε can be picked uniquely such that |ε| > 1 in this embedding. If the discriminant Δ of K satisfies |Δ|  33, then[6]

${\displaystyle \epsilon ^{3}>{\frac {|\Delta |-27}{4}}.}$

For example, the fundamental unit of ${\displaystyle \mathbf {Q} ({\sqrt[{3}]{2}})}$ is ${\displaystyle \epsilon =1+{\sqrt[{3}]{2}}+{\sqrt[{3}]{2^{2}}}}$ and ${\displaystyle \epsilon ^{3}\approx 56.9}$ whereas the discriminant of this field is −108 and

${\displaystyle {\frac {|\Delta |-27}{4}}=20.25}$

so ${\displaystyle \epsilon ^{3}>20.25}$.

## Notes

1. Alaca & Williams 2004, §13.4
2. Neukirch 1999, Exercise I.7.1
3. Alaca & Williams 2004, Table 11.5.4
4. Stevenhagen 1993, Conjecture 1.4
5. Fouvry & Klüners 2010
6. Alaca & Williams 2004, Theorem 13.6.1

## References

• Alaca, Şaban; Williams, Kenneth S. (2004), Introductory algebraic number theory, Cambridge University Press, ISBN 978-0-521-54011-7
• Duncan Buell (1989), Binary quadratic forms: classical theory and modern computations, Springer-Verlag, pp. 92–93, ISBN 978-0-387-97037-0
• Fouvry, Étienne; Klüners, Jürgen (2010), "On the negative Pell equation", Annals of Mathematics, 2 (3): 2035–2104, doi:10.4007/annals.2010.172.2035, MR 2726105
• Neukirch, Jürgen (1999), Algebraic Number Theory, Grundlehren der mathematischen Wissenschaften, 322, Berlin: Springer-Verlag, ISBN 978-3-540-65399-8, MR 1697859, Zbl 0956.11021
• Stevenhagen, Peter (1993), "The number of real quadratic fields having units of negative norm", Experimental Mathematics, 2 (2): 121–136, CiteSeerX 10.1.1.27.3512, doi:10.1080/10586458.1993.10504272, MR 1259426