Inmathematics, innumber theory,Gauss composition law is a rule, invented byCarl Friedrich Gauss, for performing abinary operation onintegralbinary quadratic forms (IBQFs). Gauss presented this rule in hisDisquisitiones Arithmeticae,^[1] a textbook onnumber theory published in 1801, in Articles 234 - 244. Gauss composition law is one of the deepest results in the theory of IBQFs and Gauss's formulation of the law and the proofs its properties as given by Gauss are generally considered highly complicated and very difficult.^[2] Several later mathematicians have simplified the formulation of the composition law and have presented it in a format suitable for numerical computations. The concept has also found generalisations in several directions.

An expression of the form${\displaystyle Q(x,y)=\alpha x^2+\beta xy+\gamma y^2}$, where${\displaystyle \alpha ,\beta ,\gamma ,x,y}$ are allintegers, is called an integral binary quadratic form (IBQF). The form${\displaystyle Q(x,y)}$ is called a primitive IBQF if${\displaystyle \alpha ,\beta ,\gamma }$ are relatively prime. The quantity${\displaystyle \mathrm{\Delta }={\beta }^2-4\alpha \gamma }$ is called the discriminant of the IBQF${\displaystyle Q(x,y)}$. An integer${\displaystyle \mathrm{\Delta }}$ is the discriminant of some IBQF if and only if${\displaystyle \mathrm{\Delta }\equiv 0,1(\mathrm{m}\mathrm{o}\mathrm{d}4)}$.${\displaystyle \mathrm{\Delta }}$ is called afundamental discriminantif and only if one of the following statements holds

• ${\displaystyle \mathrm{\Delta }\equiv 1(\mathrm{m}\mathrm{o}\mathrm{d}4)}$ and issquare-free,
• ${\displaystyle \mathrm{\Delta }=4m}$ where${\displaystyle m=2\text{ or }3(\mathrm{m}\mathrm{o}\mathrm{d}4)}$ and${\displaystyle m}$ is square-free.

If and${\displaystyle \alpha >0}$ then${\displaystyle Q(x,y)}$ is said to be positive definite; if and then${\displaystyle Q(x,y)}$ is said to be negative definite; if${\displaystyle \mathrm{\Delta }>0}$ then${\displaystyle Q(x,y)}$ is said to be indefinite.

Two IBQFs${\displaystyle g(x,y)}$ and${\displaystyle h(x,y)}$ are said to be equivalent (or, properly equivalent) if there exist integers α, β, γ, δ such that

${\displaystyle \alpha \delta -\beta \gamma =1}$ and${\displaystyle g(\alpha x+\beta y,\gamma x+\delta y)=h(x,y).}$

The notation${\displaystyle g(x,y)\sim h(x,y)}$ is used to denote the fact that the two forms are equivalent. The relation "${\displaystyle \sim }$" is an equivalence relation in the set of all IBQFs. The equivalence class to which the IBQF${\displaystyle g(x,y)}$ belongs is denoted by${\displaystyle [g(x,y)]}$.

Two IBQFs${\displaystyle g(x,y)}$ and${\displaystyle h(x,y)}$ are said to be improperly equivalent if

${\displaystyle \alpha \delta -\beta \gamma =-1}$ and${\displaystyle g(\alpha x+\beta y,\gamma x+\delta y)=h(x,y).}$

The relation in the set of IBQFs of being improperly equivalent is also an equivalence relation.

It can be easily seen that equivalent IBQFs (properly or improperly) have the same discriminant.

The following identity, calledBrahmagupta identity, was known to the Indian mathematicianBrahmagupta (598–668) who used it to calculate successively better fractional approximations to square roots of positive integers:

${\displaystyle (x^2+D{y}^2)(u^2+D{v}^2)=(xu+Dyv{)}^2+D(xv-yu{)}^2}$

Writing${\displaystyle f(x,y)=x^2+D{y}^2}$ this identity can be put in the form

${\displaystyle f(x,y)f(u,v)=f(X,Y)}$ where${\displaystyle X=xu+Dyv,Y=xv-yu}$.

Gauss's composition law of IBQFs generalises this identity to an identity of the form${\displaystyle g(x,y)h(u,v)=F(X,Y)}$ where${\displaystyle g(x,y),h(x,y),F(X,Y)}$ are all IBQFs and${\displaystyle X,Y}$ are linear combinations of the products${\displaystyle xu,xv,yu,yv}$.

Consider the following IBQFs:

${\displaystyle g(x,y)=a{x}^2+bxy+c{y}^2}$

${\displaystyle h(x,y)=d{x}^2+exy+f{y}^2}$

${\displaystyle F(x,y)=A{x}^2+Bxy+C{y}^2}$

If it is possible to find integers${\displaystyle p,q,r,s}$ and${\displaystyle p^{\mathrm{\prime }},q^{\mathrm{\prime }},r^{\mathrm{\prime }},s^{\mathrm{\prime }}}$ such that the following six numbers

${\displaystyle p{q}^{\mathrm{\prime }}-q{p}^{\mathrm{\prime }},p{r}^{\mathrm{\prime }}-r{p}^{\mathrm{\prime }},p{s}^{\mathrm{\prime }}-s{p}^{\mathrm{\prime }},q{r}^{\mathrm{\prime }}-r{q}^{\mathrm{\prime }},q{s}^{\mathrm{\prime }}-s{q}^{\mathrm{\prime }},r{s}^{\mathrm{\prime }}-s{r}^{\mathrm{\prime }}}$

have no common divisors other than ±1, and such that if we let

${\displaystyle X=pxu+qxv+ryu+syv}$

${\displaystyle Y=p^{\mathrm{\prime }}xu+q^{\mathrm{\prime }}xv+r^{\mathrm{\prime }}yu+s^{\mathrm{\prime }}yv}$

the following relation is identically satisfied

${\displaystyle g(x,y)h(u,v)=F(X,Y)}$,

then the form${\displaystyle F(x,y)}$ is said to be a composite of the forms${\displaystyle g(x,y)}$ and${\displaystyle h(x,y)}$. It may be noted that the composite of two IBQFs, if it exists, is not unique.

Consider the following binary quadratic forms:

${\displaystyle g(x,y)=2x^2+3xy-10y^2}$

${\displaystyle h(x,y)=5x^2+3xy-4y^2}$

${\displaystyle F(x,y)=10x^2+3xy-2y^2}$

Let

${\displaystyle [p,q,r,s]=[1,0,0,2],[p^{\mathrm{\prime }},q^{\mathrm{\prime }},r^{\mathrm{\prime }},s^{\mathrm{\prime }}]=[0,2,5,3]}$

We have

${\displaystyle p{q}^{\mathrm{\prime }}-q{p}^{\mathrm{\prime }}=2,p{r}^{\mathrm{\prime }}-r{p}^{\mathrm{\prime }}=5,p{s}^{\mathrm{\prime }}-s{p}^{\mathrm{\prime }}=3,q{r}^{\mathrm{\prime }}-r{q}^{\mathrm{\prime }}=0,q{s}^{\mathrm{\prime }}-s{q}^{\mathrm{\prime }}=4,r{s}^{\mathrm{\prime }}-s{r}^{\mathrm{\prime }}=10}$.

These six numbers have no common divisors other than ±1.Let

${\displaystyle X=pxu+qxv+ryu+syv=xu+2yv}$,

${\displaystyle Y=p^{\mathrm{\prime }}xu+q^{\mathrm{\prime }}xv+r^{\mathrm{\prime }}yu+s^{\mathrm{\prime }}yv=2xv+5yu+3yv}$.

Then it can be verified that

${\displaystyle g(x,y)h(u,v)=F(X,Y)}$.

Hence${\displaystyle F(x,y)}$ is a composite of${\displaystyle g(x,y)}$ and${\displaystyle h(x,y)}$.

The following algorithm can be used to compute the composite of two IBQFs.^[3]

Given the following IBQFs having the same discriminant${\displaystyle \mathrm{\Delta }}$:

${\displaystyle f_1(x,y)=a_1{x}^2+b_{1}xy+c_1{y}^2}$

${\displaystyle f_2(x,y)=a_2{x}^2+b_{2}xy+c_2{y}^2}$

${\displaystyle \mathrm{\Delta }=b_1^2-4a_1{c}_1=b_2^2-4a_2{c}_2}$

1. Compute${\displaystyle \beta =\frac{b_1+b_2}{2}}$
2. Compute${\displaystyle n=gcd(a_1,a_2,\beta )}$
3. Compute${\displaystyle t,u,v}$ such that${\displaystyle a_{1}t+a_{2}u+\beta v=n}$
4. Compute${\displaystyle A=\frac{a_1{a}_2}{n^2}}$
5. Compute${\displaystyle B=\frac{a_1{b}_{2}t+a_2{b}_{1}u+v(b_1{b}_2+\mathrm{\Delta })/2}{n}}$
6. Compute${\displaystyle C=\frac{B^2-\mathrm{\Delta }}{4A}}$
7. Compute${\displaystyle F(x,y)=A{x}^2+Bxy+C{y}^2}$
8. Compute

${\displaystyle X=n{x}_1{x}_2+\frac{(b_2-B)n}{2a_2}{x}_1{y}_2+\frac{(b_1-B)n}{2a_1}{y}_1{x}_2+\frac{[b_1{b}_2+\mathrm{\Delta }-B(b_1+b_2)]n}{4a_1{a}_2}{y}_1{y}_2}$

${\displaystyle Y=\frac{a_1}{n}{x}_1{y}_2+\frac{a_2}{n}{y}_1{x}_2+\frac{b_1+b_2}{2n}{y}_1{y}_2}$

Then${\displaystyle F(X,Y)=f_1(x_1,y_1)f_2(x_2,y_2)}$ so that${\displaystyle F(x,y)}$ is a composite of${\displaystyle f_1(x,y)}$ and${\displaystyle f_2(x,y)}$.

The composite of two IBQFs exists if and only if they have the same discriminant.

Let${\displaystyle g(x,y),h(x,y),g^{\mathrm{\prime }}(x,y),h^{\mathrm{\prime }}(x,y)}$ be IBQFs and let there be the following equivalences:

${\displaystyle g(x,y)\sim g^{\mathrm{\prime }}(x,y)}$

${\displaystyle h(x,y)\sim h^{\mathrm{\prime }}(x,y)}$

If${\displaystyle F(x,y)}$ is a composite of${\displaystyle g(x,y)}$ and${\displaystyle h(x,y)}$, and${\displaystyle F^{\mathrm{\prime }}(x,y)}$ is a composite of${\displaystyle g^{\mathrm{\prime }}(x,y)}$ and${\displaystyle h^{\mathrm{\prime }}(x,y)}$, then

${\displaystyle F(x,y)\sim F^{\mathrm{\prime }}(x,y).}$

Let${\displaystyle D}$ be a fixed integer and consider set${\displaystyle S_D}$ of all possible primitive IBQFs of discriminant${\displaystyle D}$. Let${\displaystyle G_D}$ be the set of equivalence classes in this set under the equivalence relation "${\displaystyle \sim }$". Let${\displaystyle [g(x,y)]}$ and${\displaystyle [h(x,y)]}$ be two elements of${\displaystyle G_D}$. Let${\displaystyle F(x,y)}$ be a composite of the IBQFs${\displaystyle g(x,y)}$ and${\displaystyle h(x,y)}$ in${\displaystyle S_D}$. Then the following equation

${\displaystyle [g(x,y)]\circ [h(x,y)]=[F(x,y)]}$

defines a well-defined binary operation "${\displaystyle \circ }$" in${\displaystyle G_D}$.

• The set${\displaystyle G_D}$ is a finite abelian group under the binary operation${\displaystyle \circ }$.

• The identity element in the group${\displaystyle G_D}$ =

• The inverse of${\displaystyle [a{x}^2+bxy+c{y}^2]}$ in${\displaystyle G_D}$ is${\displaystyle [a{x}^2-bxy+c{y}^2]}$.

The following sketch of the modern approach to the composition law of IBQFs is based on a monograph by Duncan A. Buell.^[4] The book may be consulted for further details and for proofs of all the statements made hereunder.

Let${\displaystyle \mathbb{Z}}$ be the set of integers. Hereafter, in this section, elements of${\displaystyle \mathbb{Z}}$ will be referred asrational integers to distinguish them fromalgebraic integers to be defined below.

A complex number${\displaystyle \alpha }$ is called aquadraticalgebraic number if it satisfies an equation of the form

${\displaystyle a{x}^2+bx+c=0}$ where${\displaystyle a,b,c\in \mathbb{Z}}$.

${\displaystyle \alpha }$ is called aquadratic algebraic integer if it satisfies an equation of the form

${\displaystyle x^2+bx+c=0}$ where${\displaystyle b,c\in \mathbb{Z}}$

The quadratic algebraic numbers are numbers of the form

${\displaystyle \alpha =\frac{-b+e\sqrt{d}}{2a}}$ where${\displaystyle a,b,d,e\in \mathbb{Z}}$ and${\displaystyle d}$ has no square factors other than${\displaystyle 1}$.

The integer${\displaystyle d}$ is called theradicand of the algebraic integer${\displaystyle \alpha }$. Thenorm of the quadratic algebraic number${\displaystyle \alpha }$ is defined as

${\displaystyle N(\alpha )=(b^2+e^{2}d)/4a^2}$.

Let${\displaystyle \mathbb{Q}}$ be the field of rational numbers. The smallest field containing${\displaystyle \mathbb{Q}}$ and a quadratic algebraic number${\displaystyle \alpha }$ is thequadratic field containing${\displaystyle \alpha }$ and is denoted by${\displaystyle \mathbb{Q}(\alpha )}$. This field can be shown to be

${\displaystyle \mathbb{Q}(\alpha )=\mathbb{Q}(\sqrt{d})=\{t+u\sqrt{d}|t,u\in \mathbb{Q}\}}$

Thediscriminant${\displaystyle \mathrm{\Delta }}$ of the field${\displaystyle \mathbb{Q}(\sqrt{d})}$ is defined by

Let${\displaystyle d\ne 1}$ be a rational integer without square factors (except 1). The set of quadratic algebraic integers of radicand${\displaystyle d}$ is denoted by${\displaystyle O(\sqrt{d})}$. This set is given by

${\displaystyle O(\sqrt{d})}$ is a ring under ordinary addition and multiplication. If we let

then

${\displaystyle O(\sqrt{d})=\{a+b\delta |a,b\in \mathbb{Z}\}}$.

Let${\displaystyle \mathbf{a}}$ be anideal in the ring of integers${\displaystyle O(\sqrt{d})}$; that is, let${\displaystyle \mathbf{a}}$ be a nonempty subset of${\displaystyle O(\sqrt{d})}$ such that for any${\displaystyle \alpha ,\beta \in \mathbf{a}}$ and any${\displaystyle \lambda ,\mu \in O(\sqrt{d})}$,${\displaystyle \lambda \alpha +\mu \beta \in \mathbf{a}}$. (An ideal${\displaystyle \mathbf{a}}$ as defined here is sometimes referred to as anintegral ideal to distinguish fromfractional ideal to be defined below.) If${\displaystyle \mathbf{a}}$ is an ideal in${\displaystyle O(\sqrt{d})}$ then one can find${\displaystyle {\alpha }_1,{\alpha }_2\in O(\sqrt{d})}$ such any element in${\displaystyle \mathbf{a}}$ can be uniquely represented in the form${\displaystyle {\alpha }_{1}x+{\alpha }_{2}y}$ with${\displaystyle x,y\in \mathbb{Z}}$. Such a pair of elements in${\displaystyle O(\sqrt{d})}$ is called abasis of the ideal${\displaystyle \mathbf{a}}$. This is indicated by writing${\displaystyle \mathbf{a}=⟨{\alpha }_1,{\alpha }_2⟩}$. Thenorm of${\displaystyle \mathbf{a}=⟨{\alpha }_1,{\alpha }_2⟩}$ is defined as

${\displaystyle N(\mathbf{a})=|{\alpha }_1\overline{{\alpha }_2}-\overline{{\alpha }_1}{\alpha }_2|/\sqrt{\mathrm{\Delta }}}$.

The norm is independent of the choice of the basis.

• Theproduct of two ideals${\displaystyle \mathbf{a}=⟨{\alpha }_1,{\alpha }_2⟩}$ and${\displaystyle \mathbf{b}=⟨{\beta }_1,{\beta }_2⟩}$, denoted by${\displaystyle \mathbf{a}\mathbf{b}}$, is the ideal generated by the${\displaystyle \mathbb{Z}}$-linear combinations of${\displaystyle {\alpha }_1{\beta }_1,{\alpha }_1{\beta }_2,{\alpha }_2{\beta }_1,{\alpha }_2{\beta }_2}$.

• Afractional ideal is a subset${\displaystyle I}$ of the quadratic field${\displaystyle \mathbb{Q}(\sqrt{\mathrm{\Delta }})}$ for which the following two properties hold:

1. For any${\displaystyle \alpha ,\beta \in I}$ and for any${\displaystyle \lambda ,\mu \in O(\sqrt{d})}$,${\displaystyle \lambda \alpha +\mu \beta \in I}$.
2. There exists a fixed algebraic integer${\displaystyle \nu }$ such that for every${\displaystyle \alpha \in I}$,${\displaystyle \nu \alpha \in O(\sqrt{d})}$.

• An ideal${\displaystyle \mathbf{a}}$ is called aprincipal ideal if there exists an algebraic integer${\displaystyle \alpha }$ such that${\displaystyle \mathbf{a}=\{\lambda \alpha |\lambda \in O(\sqrt{d})\}}$. This principal ideal is denoted by${\displaystyle (\alpha )}$.

There is this important result: "Given any ideal (integral or fractional)${\displaystyle \mathbf{a}}$, there exists an integral ideal${\displaystyle \mathbf{b}}$ such that the product ideal${\displaystyle \mathbf{a}\mathbf{b}}$ is a principal ideal."

Two (integral or fractional) ideals${\displaystyle \mathbf{a}}$ and${\displaystyle \mathbf{b}}$ are said to beequivalent, denoted${\displaystyle \mathbf{a}\sim \mathbf{b}}$, if there is a principal ideal${\displaystyle (\alpha )}$ such that${\displaystyle \mathbf{a}=(\alpha )\mathbf{b}}$. These ideals arenarrowly equivalent if the norm of${\displaystyle \alpha }$ is positive. The relation, in the set of ideals, of being equivalent or narrowly equivalent as defined here is indeed an equivalence relation.

The equivalence classes (respectively, narrow equivalence classes) of fractional ideals of a ring of quadratic algebraic integers${\displaystyle O(\sqrt{d})}$ form an abelian group under multiplication of ideals. The identity of the group is the class of all principal ideals (respectively, the class of all principal ideals${\displaystyle (\alpha )}$ with${\displaystyle N(\alpha )>0}$). The groups of classes of ideals and of narrow classes of ideals are called theclass group and thenarrow class group of the${\displaystyle \mathbb{Q}(\sqrt{d})}$.

The main result that connects the IBQFs and classes of ideals can now be stated as follows:

"The group of classes of binary quadratic forms of discriminant${\displaystyle \mathrm{\Delta }}$ is isomorphic to the narrow class group of the quadratic number field${\displaystyle \mathbb{Q}(\sqrt{\mathrm{\Delta }})}$."

Manjul Bhargava, a Canadian-American Fields Medal winning mathematician introduced a configuration, called aBhargava cube, of eight integers${\displaystyle a,b,c,d,e,f}$ (see figure) to study the composition laws of binary quadratic forms and other such forms. Defining matrices associated with the opposite faces of this cube as given below

,

Bhargava constructed three IBQFs as follows:

${\displaystyle Q_1=-det(M_{1}x+N_{1}y),Q_2=-det(M_{2}x+N_{2}y)Q_3=-det(M_{3}x+N_{3}y)}$

Bhargava established the following result connecting a Bhargava cube with the Gauss composition law:^[5]

"If a cube A gives rise to three primitive binary quadratic formsQ₁,Q₂,Q₃, thenQ₁,Q₂,Q₃ have the same discriminant, and the product of these three forms is the identity in the group defined by Gauss composition. Conversely, ifQ₁,Q₂,Q₃ are any three primitive binary quadratic forms of the same discriminant whose product is the identity under Gauss composition, then there exists a cube A yieldingQ₁,Q₂,Q₃."

• Carl Friedrich Gauss
• Quadratic forms
• Number theory