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Arithmetic in Extensions of $$\mathbb Q$$ (Table of contents)

## Introduction to Extensions of $$\mathbb Q$$

What makes work with rational numbers and integers comfortable are the essential properties they have, especially the unique factorization property (the Main Theorem of Arithmetic). However, the might of the arithmetic in $$\mathbb{Q}$$ is bounded. Thus, some polynomials, although they have zeros, cannot be factorized into polynomials with rational coefficients. Nevertheless, such polynomials can always be factorized in a wider field. For instance, the polynomial $$x^2+1$$ is irreducible over $$\mathbb{Z}$$ or $$\mathbb{Q}$$, but over the ring of the so called Gaussian integers $$\mathbb Z [i]=\{a+b i \mid a,b\in\mathbb Z \}$$ it can be factorized as $$(x+i)(x-i)$$. Sometimes the wider field retains many properties of the rational numbers. In particular, it will turn out that the Gaussian integers are a unique factorization domain, just like the (rational) integers $$\mathbb{Z}$$. We shall first discuss some basics of higher algebra.

Definition 1.

A number $$\alpha\in\mathbb{C}$$ is algebraic if there is a polynomial $$p(x)=a_nx^n+a_{n-1} x^{n-1}+\dots+a_0$$ with integer coefficients such that $$p(\alpha)=0$$. If $$a_n=1$$, then $$\alpha$$ is an algebraic integer.

Further, $$p(x)$$ is the minimal polynomial of $$\alpha$$ if it is irreducible over $$\mathbb{Z}[x]$$ (i.e. it cannot be written as a product of nonconstant polynomials with integer coefficients).

Example 1. The number $$i$$ is an algebraic integer, as it is a root of the polynomial $$x^2+1$$ which is also its minimal polynomial. Number $$\sqrt2+\sqrt3$$ is also an algebraic integer with the minimal polynomial $$x^4-10x^2+1$$ (verify!).

Example 2. The minimal polynomial of a rational number $$q=a/b$$ ($$a\in\mathbb{Z}$$, $$b\in\mathbb{N}$$, $$(a,b)=1$$) is $$bx-a$$. By the definition, $$q$$ is an algebraic integer if and only if $$b=1$$, i.e. if and only if $$q$$ is an integer.

Definition 2. Let $$\alpha$$ be an algebraic integer and $$p(x)=x^n+a_{n-1}x^{n-1}+\dots+a_0$$ ($$a_i\in\mathbb{Z}$$) be its minimal polynomial. The extension of a ring $$A$$ by the element $$\alpha$$ is the set $$A[\alpha]$$ of all complex numbers of the form $c_0+c_1\alpha+\dots+ c_{n-1}\alpha^{n-1}\;\;(c_i\in A),\quad\quad\quad\quad\quad(\ast)$ with all the operations inherited from $$A$$. The degree of the extension is the degree $$n$$ of the polynomial $$p(x)$$.

The theme of this text are extensions of the ring $$\mathbb{Z}$$ of degree 2, so called quadratic extensions. Thus, for example, the polynomials $$x^2+1$$ and $$x^2+x+1$$ determine the extensions $$\mathbb{Z}[i]$$ and $$\mathbb{Z}[\omega]$$, where $$\omega=\frac{-1+i\sqrt3}2$$ (this notation will be used later).

All elements of a quadratic extension of $$\mathbb{Z}$$ are algebraic integers with the minimal polynomial of second degree. Two elements having the same minimal polynomials are said to be conjugates. Each nonrational element $$z$$ of the quadratic extension has exactly one conjugate, called the conjugate of $$z$$ and denoted $$\overline{z}$$. For a rational integer $$z$$ we define $$\overline{z}=z$$.

Definition 3. The norm of an element $$z$$ of a quadratic extension of $$\mathbb{Z}$$ is $$N(z)=z\overline{z}$$.

The norm is always an integer. Roughly speaking, it is a kind of equivalent of the absolute value in the set of integers $$\mathbb{Z}$$.

Definition 4.

If $$z\in\mathbb{Z}[\sqrt{d}]$$, $$z=a+b\sqrt{d}$$ ($$a,b\in\mathbb{Z}$$), then $$\overline{z}=a-b\sqrt{d}$$ and $$N(z)=a^2-db^2$$. In particular, in $$\mathbb{Z}[i]$$ the norm of element $$a+bi$$ ($$a,b\in\mathbb{N}$$) is $$N(a+bi)=a^2+b^2$$.

If $$z=a+b\omega\in\mathbb{Z}[\omega]$$ ($$a,b\in\mathbb{Z}$$), then $$\overline{z}=a-b-b\omega$$ and $$N(z)=a^2-ab+b^2$$.

In every complex quadratic field the conjugation corresponds to the complex conjugation.

The following two propositions follow directly from definition.

Theorem 1.

The norm is multiplicative, i.e. for arbitrary elements $$z_1,z_2$$ of a quadratic extension of $$\mathbb{Z}$$ it holds that $$N(z_1z_2)=N(z_1)N(z_2)$$.

An element $$\epsilon\in\mathbb{Z}[\alpha]$$ is called a unit if there exists $$\epsilon^{\prime}\in\mathbb{Z}[\alpha]$$ such that $$\epsilon\epsilon^{\prime}=1$$. In that case $$N(\epsilon)N(\epsilon^{\prime})=N(1)=1$$, so $$N(\epsilon)=\pm1$$. In fact, $$\epsilon$$ is a unit if and only if its norm is $$\pm1$$: indeed, if $$N(\epsilon)=\pm1$$ then $$\epsilon\overline{\epsilon}=\pm1$$ by definition.

Theorem 2. The norm is multiplicative, i.e. for arbitrary elements $$z_1,z_2$$ of a quadratic extension of $$\mathbb{Z}$$ it holds that $$N(z_1z_2)=N(z_1)N(z_2)$$.

An element $$\epsilon\in\mathbb{Z}[\alpha]$$ is called a unit if there exists $$\epsilon^{\prime}\in\mathbb{Z}[\alpha]$$ such that $$\epsilon\epsilon^{\prime}=1$$. In that case $$N(\epsilon)N(\epsilon^{\prime})=N(1)=1$$, so $$N(\epsilon)=\pm1$$. In fact, $$\epsilon$$ is a unit if and only if its norm is $$\pm1$$: indeed, if $$N(\epsilon)=\pm1$$ then $$\epsilon\overline{\epsilon}=\pm1$$ by definition.

Example 3.

The only units in $$\mathbb{Z}$$ are $$\pm1$$.

If $$a+bi$$ ($$a,b\in\mathbb{Z}$$) is a unit, then $$N(a+bi)=a^2+b^2=\pm1$$, which implies $$a+bi\in\{\pm1,\pm i\}$$.

All units in $$\mathbb{Z}[\omega]$$ are $$\pm1,\pm\omega, \pm(1+\omega)$$. Indeed, if $$a+b\omega$$ is a unit then $$a^2-ab+b^2=1$$, i.e. $$(2a-b)^2+3b^2=4$$ and he result follows. Note that $$\omega^2$$ equals $$-(1+\omega)$$.

Problem 1.

Let $$p$$ be a prime number and $$N=\prod _{k=1}^{p-1}(k^2+1)$$. Determine the remainder of $$N$$ upon division by $$p$$.

The divisibility and congruences in an extension $$K$$ of the ring $$\mathbb{Z}$$ is defined in the usual way: $$x\in K$$ is divisible by $$y\in K$$ (denoted $$y\mid x$$) if there exists $$z\in K$$ such that $$x=yz$$, and $$x\equiv y$$ (mod $$z$$) if $$z\mid x-y$$.

Since every element of a quadratic ring is divisible by every unit, the definition of the notion of a prime must be adjusted to the new circumstances.

Definition 5. An element $$y$$ of a quadratic ring $$K$$ is adjoint to element $$x$$ (denoted $$y\sim x$$) if there exists a unit $$\epsilon$$ such that $$y=\epsilon x$$.

Definition 6. A nonzero element $$x\in K$$ which is not a unit is prime if it has no other divisors but the units and elements adjoint to itself.

We have the following simple proposition.

Theorem 3. Let $$x\in K$$. If $$N(x)$$ is a prime, then $$x$$ is prime.

The converse does not hold, as $$3$$ is a prime in $$\mathbb{Z}[i]$$, but $$N(3)=9$$ is composite.

Of course, the elements conjugate or adjoint to a prime are also primes. Therefore the smallest positive rational integer divisible by a prime $$z$$ equals $$z\overline{z}=N(z)$$.

Consider an arbitrary nonzero and nonunit element $$x\in K$$. If $$x$$ is not prime then there are nonunit elements $$y,z\in K$$ such that $$yz=x$$. Hereby $$N(y)N(z)=N(x)$$ and $$N(y),N(z) > 1$$. Hence $$N(y),N(z) < N(x)$$. Continuing this procedure we end up with a factorization $$x=x_1x_2\cdots x_k$$ in which all elements are prime. Therefore, we have

Theorem 4. Every nonzero and nonunit $$x\in K$$ can be factorized into primes.

Problem 2. Given a nonzero and nonunit element $$z\in K$$, find the number of equivalence classes in $$K$$ modulo $$z$$.

Naturally, we would like to know when the factorization into primes is unique, i.e. when the Fundamental Theorem of Arithmetic holds. But let us first note that, by the above definition, the primes of $$\mathbb{Z}$$ are $$\pm2,\pm3,\pm5$$, etc, so the factorization into primes is not exactly unique, as e.g. $$2\cdot3=(-2)(-3)$$. Actually, in this case the uniqueness of factorization is true in the following wording.

Definition 7. FTA, or "The Fundamental Theorem of Arithmetic" means: Each nonzero and nonunit element of $$\mathbb{Z}$$ or of its quadratic extension $$K$$ can be written as a product of primes. This factorization is unique up to the order of the factors and adjoining between corresponding factors.

The division with remainder in a quadratic extension $$K$$ of $$\mathbb{Z}$$ can be formulated as follows:

Definition 8. DWR means: For each $$a,b\in K$$ with $$b\neq0$$ there exist $$p,q\in K$$ such that $$a=pb+q$$ and $$N(q) < N(b)$$.

Obviously, such a division, if it exists, is not necessarily unique - it is not so even in $$\mathbb{Z}$$ itself. Moreover, it does not exist in some quadratic extensions, as we shall see later. The significance of the existence of a division with remainder, however, lies in the fact that it implies the uniqueness of factorization:

Theorem 5. If the division with remainder in a quadratic ring $$K$$ is always possible, then FTA holds in $$K$$.

There are quadratic rings in which FTA holds despite the nonexistence of a division with remainder. However, FTA is an exception rather than a rule.

Example 4. FTA is false in $$\mathbb{Z}[\sqrt{-5}]$$, as 9 has two factorizations into primes: $$9=3\cdot3=(2+\sqrt{-5})(2-\sqrt{-5})$$, which are not equivalent since $$2\pm\sqrt{-5}\not\sim3$$.

Example 5. The factorizations of the element $$4-\omega$$ in $$\mathbb{Z}[\omega]$$ as $$(1-\omega)(3+\omega)=(-2-3\omega)(1+2\omega)$$ are considered the same, since $$1+2\omega=\omega(1-\omega)\sim 1-\omega$$ and $$-2-3\omega=-(1+\omega)(3+\omega)\sim 3+\omega$$. We shall show later that FTA is true in $$\mathbb{Z}[\omega]$$.

# Arithmetics in Gaussian Integers $$\mathbb Z[i]$$

We have already seen that the norm of element $$a+bi\in\mathbb{Z}[i]$$ ($$a,b\in\mathbb{Z}$$) is $$N(a+bi)=a^2+b^2$$ and the units are $$\pm1$$ and $$\pm i$$. Therefore, all divisors of a prime element $$\pi\in\mathbb{Z}[i]$$ are $$\pm1,\pm i,\pm\pi,\pm i\pi$$.

Theorem 6 The Fundamental Theorem of Arithmetic (FTA) holds in the set of Gaussian integers $$\mathbb{Z}[i]$$.

Based on theorem 5, it is enough to show that for all $$a,b\in\mathbb{Z}[i]$$ with $$b\neq0$$ there exists an element $$p\in\mathbb{Z}[i]$$ such that $$N(a-pb) < N(b)$$.

Let $$\sigma,\tau\in\mathbb{R}$$ be such that $$a/b=\sigma+\tau i$$, and let $$s,t\in\mathbb{Z}$$ be such that $$|\sigma-s|\leq 1/2$$ and $$|\tau-t|\leq1/2$$. Setting $$p=s+ti$$ yields $$a-pb=(\sigma+\tau i)b-pb=[(\sigma-s)+(\tau-t)i]b$$, which implies $N(a-pb)= N[(\sigma-s)+(\tau-t)i]N(b)=[(\sigma-s)^2+(\tau-t)^2]N(b)\leq N(b)/2 < N(b).$ This proves the theorem.

The following proposition describes all prime elements in the set of Gaussian integers.

Theorem 7 An element $$x\in\mathbb{Z}[i]$$ is prime if and only if $$N(x)$$ is a prime or $$|x|$$ is a prime integer of the form $$4k-1$$, $$k\in\mathbb{N}$$.

Consider an arbitrary prime $$x=a+bi \in\mathbb{Z}[i]$$ ($$a,b\in \mathbb{Z}$$). Element $$\overline{x}$$ is prime also (indeed, if $$\overline{x}=yz$$, then $$x=\overline{y}\: \overline{z}$$), so $$N(x)$$ factorizes into primes as $$x\overline{x}$$.

Suppose that $$N(x)$$ is composite, $$N(x)=mn$$ for some two natural numbers $$m,n > 1$$. It follows from $$x\overline{x}=mn$$ and the FTA that $$x\sim m$$ or $$x\sim n$$, so we may suppose w.l.o.g. that $$x$$ is a prime integer. If $$x=2$$ or $$x\equiv1$$ (mod 4), then there exist integers $$a,b\in\mathbb{Z}$$ such that $$N(a+bi)=(a+bi)(a-bi)=a^2+b^2=x$$; hence $$x$$ is composite in $$\mathbb{Z}[i]$$. On the other hand, if $$x$$ is a prime integer with $$x\equiv3$$ {\rm(mod 4)}, then $$x$$ is also prime in $$\mathbb{Z}[i]$$. Indeed, if $$x=uv$$ for some nonunit elements $$u,v\in\mathbb{Z}[i]$$, then $$x^2=N(x)=N(u)N(v)$$ implies $$N(u)=N(v)=x$$ which is impossible. This proves our claim.

Problem 3 Solve the equation $$x^5-1=y^2$$ in integers.

Rewrite the equation in the form $$x^5=(y+i)(y-i)$$. Note that $$x$$ is not even, as otherwise $$y^2\equiv-1$$ (mod 4). Thus $$y$$ is even and consequently the elements $$y+i$$ and $$y-i$$ are coprime in $$\mathbb{Z}[i]$$. Since $$(y+i)(y-i)$$ is a fifth power, it follows that $$y+i$$ and $$y-i$$ are both fifth powers. Let $$a,b\in \mathbb{Z}$$ be such that $$y+i=(a+bi)^5=a(a^4-10a^2b^2+5b^4)+b(5a^4-10a^2b^2+b^4)i$$. It holds that $$b(5a^4-10a^2b^2+b^4)=1$$, and therefore $$b=\pm1$$. It is easily seen that we must have $$y=0$$ and $$x=1$$.

Problem 4 Suppose that $$x,y$$ and $$z$$ are natural numbers satisfying $$xy=z^2+1$$. Prove that there exist integers $$a,b,c,d$$ such that $$x=a^2+b^2$$, $$y=c^2+d^2$$ and $$z=ac+bd$$.

We use the following important fact: If $$m,n,p,q$$ are elements of a unique factorization domain $$K$$ (in this case, $$K=\mathbb{Z}[i]$$) satisfying $$mn=pq$$, then there exist $$u_1,u_2,v_1,v_2\in K$$ such that $$m=u_1v_1$$, $$n=u_2v_2$$, $$p=u_1v_2$$, $$q=u_2v_1$$. The proof of this fact is the same as in the case of $$m,n,p,q\in\mathbb{Z}$$ and follows directly from the factorizations of $$m,n,p,q$$ into primes.

Since $$xy=z^2+1=(z+i)(z-i)$$, the above fact gives us $\begin{array}{cccccccc}x=u_1v_1&(1), &y=u_2v_2&(2),&z+i=u_1v_2&(3),&z-i=u_2v_1&(4)\end{array}$ for some $$u_1,u_2,v_1,v_2\in\mathbb{Z}[i]$$. The numbers $$x,y$$ are real, and therefore $$v_1=q_1\overline{u_1}$$, $$v_2=q_2\overline{u_2}$$ for some rational numbers $$q_1,q_2$$. From (3) and (4) we easily conclude that $$q_1=q_2=1$$. Now putting $$u_1=a+bi$$, $$u_2=c+di$$ yields $$x=u_1\overline{u_1}=a^2+b^2$$, $$y=c^2+d^2$$ and $$z=ac+bd$$.

Here $$\omega$$ denotes a primitive cubic root of unity. Then the norm of an element $$a+b\omega\in\mathbb{Z}[\omega]$$ ($$a,b\in\mathbb{Z}$$) is $$N(a+b\omega)=a^2-ab+b^2$$ and the units are $$\pm1$$, $$\pm \omega$$ and $$\pm(1+\omega)=\mp\omega^2$$.

Theorem 8 FTA holds in the ring $$\mathbb{Z}[\omega]$$.

By the theorem 5, it suffices to show that a division with remainder is possible, i.e. for all $$a,b\in\mathbb{Z}[\omega]$$, $$b\neq0$$ there exist $$p\in\mathbb{Z}[\omega]$$ such that $$N(a-pb) < N(b)$$.

Like in the proof for the Gaussian integers, let $$\sigma,\tau\in\mathbb{R}$$ be such that $$a/b=\sigma+\tau i$$, and let $$s,t\in\mathbb{Z}$$ be such that $$|\sigma-s|\leq 1/2$$ and $$|\tau-t|\leq1/2$$. Setting $$p=s+ti$$ gives us $$N(a-pb)\leq 3N(b)/4 < N(b),$$ implying the theorem.

Problem 5 If $$p\equiv1$$ (mod 6) is a prime number, prove that there exist $$a,b\in\mathbb{Z}$$ such that $$p=a^2-ab+b^2$$.

It suffices to show that $$p$$ is composite in $$\mathbb{Z}[\omega]$$. Indeed, if there is a prime element $$z=a+b\omega\in\mathbb{Z}[\omega]$$ ($$a,b\in\mathbb{Z}$$) that divides $$p$$, then also $$\overline{z}\mid\overline{p}=p$$. Note that $$z$$ and $$\overline{z}$$ are coprime; otherwise $$z\mid\overline{z}$$, so there exists a unit element $$\epsilon$$ with $$\overline{z}=\epsilon z$$, and hence $$z\sim (1-\omega)\mid 3$$, which is false. Therefore $$a^2-ab+b^2=z\overline{z}\mid p$$, which implies $$a^2-ab+b^2=p$$.

Thus we need to prove that $$p$$ is composite in $$\mathbb{Z}[\omega]$$. It follows from the condition on $$p$$ that -3 is a quadratic residue modulo $$p$$, so there exist $$m,n\in\mathbb{Z}$$ which are not divisible by $$p$$ such that $$p\mid(2m-n)^2+3n^2=4(m^2-mn+n^2)$$, i.e. $$p\mid(m-n\omega) \overline{m-n\omega}$$. However, $$p$$ does not divide any of the elements $$(m-n\omega),\overline{m-n\omega}$$, so it must be composite.

Theorem 9 Element $$x\in\mathbb{Z}[\omega]$$ is prime if and only if $$N(x)$$ is prime or $$|x|$$ is a prime integer of the form $$3k-1$$, $$k\in\mathbb{N}$$.

Number $$x=3$$ is composite, as $$N(1-\omega)= (1-\omega)(2+\omega)=3$$. Moreover, by problem 4, every prime integer $$p\equiv1$$ (mod 6) is composite in $$\mathbb{Z}[\omega]$$.

The rest of the proof is similar to the proof of Theorem 7 and is left as an exercise.

Maybe the most famous application of the elementary arithmetic of the ring $$\mathbb{Z}[\omega]$$ is the Last Fermat Theorem for the exponent $$n=3$$. This is not unexpected, having in mind that $$x^3+y^3$$ factorizes over $$\mathbb{Z}[\omega]$$ into linear factors: $x^3+y^3=(x+y)(x+\omega y)(x+\omega^2y)=(x+y)(\omega x+\omega^2y) (\omega^2x+\omega y).\quad\quad\quad\quad\quad(1)$ The proof we present was first given by Gauss.

Theorem 10 The equation $x^3+y^3=z^3\quad\quad\quad\quad\quad(\ast)$ has no nontrivial solutions in $$\mathbb{Z}[\omega]$$, and consequently has none in $$\mathbb{Z}$$ either.

Suppose that $$x,y,z$$ are nonzero elements of $$\mathbb{Z}[\omega]$$ that satisfy $$(\ast)$$. We can assume w.l.o.g. that $$x,y,z$$ are pairwise coprime.

Consider the number $$\rho=1-\omega$$. It is prime, as its norm equals $$(1-\omega)(1-\omega^2)=3$$. We observe that $$\overline{\rho}=1-\omega^2=(1-\omega)(1+\omega)\sim\rho$$; hence $$\alpha\in\mathbb{Z}[\omega]$$ is divisible by $$\rho$$ if and only if so is $$\overline{alpha}$$. Each element in $$\mathbb{Z}[\omega]$$ is congruent to $$-1,0$$ or 1 (mod $$\rho$$): indeed, $$a+b\omega\equiv a+b=3q+r\equiv r$$ (mod $$\rho$$) for some $$q\in\mathbb{Z}$$ and $$r\in\{-1,0,1\}$$.

The importance of number $$\rho$$ lies in the following property: $\alpha\equiv\pm1\; (\mbox{mod }\rho)\; (\alpha\in\mathbb{Z}[\omega])\;\;\mbox{implies}\;\; \alpha^3\equiv\pm1\;(\mbox{mod }\rho^4). \quad\quad\quad\quad\quad(2)$ Indeed, if $$\alpha=\pm1+\beta\rho$$, we have $$a^3\mp1=(a\mp1)(a\mp\omega) (a\mp\omega^2)=\rho^3\beta(\beta\pm1)(\beta\pm(1+\omega))$$, where the elements $$b$$, $$b\pm1$$, $$b\pm(1+\omega)$$ leave three distinct remainders modulo $$\rho$$, implying that one of them is also divisible by $$\rho$$, thus justifying our claim.

Among the numbers $$x,y,z$$, (exactly) one must be divisible by $$\rho$$: otherwise, by (2), $$x^3,y^3,z^3$$ would be congruent to $$\pm1$$ (mod $$\rho^4$$), which would imply one of the false congruences $$0\equiv\pm1$$, $$\pm1\equiv\pm2$$ (mod $$\rho^4$$). We assume w.l.o.g. that $$\rho\mid z$$. Moreover, (2) also gives us that $$\rho^2\mid z$$.

Let $$k\geq2$$ be the smallest natural number for which there exists a solution to $$(\ast)$$ with $$(x,y,z)=1$$ and $$\rho^k\mid z$$, $$\rho^{k+1}\nmid z$$. Consider this solution $$(x,y,z)$$.

The factors $$x+y$$, $$\omega x+\omega^2y$$, $$\omega^2x+\omega y$$ from (1) are congruent modulo $$\rho$$ and have the sum 0. It follows from $$\rho\mid z$$ that each of them is divisible by $$\rho$$ and that $$\rho$$ is their greatest common divisor. Let $x+y=A\rho,\;\;\;\omega x+\omega^2y=B\rho,\;\;\;\omega^2x+\omega y=C\rho,$ where $$A,B,C\in\mathbb{Z}[\omega]$$ are pairwise coprime and $$A+B+C=0$$. The product $$ABC$$ is a perfect cube (equal to $$(z/\rho)^3$$), and hence each of $$A,B,C$$ is adjoint to a cube: $A=\alpha\zeta^3,\;\;\;B=\beta\eta^3,\;\;\;C=\gamma\xi^3$ for some pairwise coprime $$\zeta,\eta,\xi\in\mathbb{Z}[\omega]$$ and units $$\alpha,\beta,\gamma$$. Therefore, $\alpha\zeta^3+\beta\eta^3 +\gamma\xi^3=0.\quad\quad\quad\quad\quad(3)$ Since $$\alpha\beta\gamma$$ is a unit and a perfect cube, we have $$\alpha\beta\gamma=\pm1$$. Furthermore, $$ABC=(z/\rho)^3$$ is divisible by $$\rho$$ (since $$\rho^2\mid z$$), so (exactly) one of the numbers $$\zeta,\eta,\xi$$, say $$\xi$$, is divisible by $$\rho$$. In fact, $$\xi^3$$ divides $$ABC$$ which is divisible by $$\rho^{3k-3}$$ and not by $$\rho^{3k-2}$$, so $$\rho^{k-1}$$ is the greatest power of $$\rho$$ that divides $$\xi$$. The numbers $$\zeta$$ and $$\eta$$ are not divisible by $$\rho$$; consequently, $$\zeta^3$$ and $$\eta^3$$ are congruent to $$\pm1$$ modulo $$\rho^4$$. Thus the equality $$A+B+C=0$$ gives us $$\alpha\pm\beta\equiv0$$ (mod $$\rho^4$$), therefore $$\beta=\pm \alpha$$; now $$\alpha\beta\gamma=\pm1$$ implies $$\gamma=\pm\alpha$$.

Canceling $$\alpha$$ in equation (3) yields $$\zeta^3\pm\eta^3\pm\xi^3=0$$, which gives us another nontrivial solution to $$(\ast)$$ with $$(\zeta,\eta,\xi)=1$$. However, in this solution we have $$\rho^{k-1}\mid\xi$$ and $$\rho^k\nmid\xi$$, which contradicts the choice of $$k$$.

# Arithmetics in Other Quadratic Rings

Every quadratic ring belongs to one of the two classes:

• $$1^{\circ}$$ Extensions of the form $$K=\mathbb{Z}[\sqrt{d}]$$, where $$d\neq1$$ is a squarefree integer. The conjugation and norm are given by the formulas $$\overline{x+y\sqrt{d}}=x-y\sqrt{d}$$ and $$N(x+y\sqrt{d})=x^2-dy^2$$, where $$x,y\in\mathbb{Z}$$.
• $$2^{\circ}$$ Extensions of the form $$K=\mathbb{Z}[\alpha]$$ for $$\alpha=\frac{-1+\sqrt{d}}2$$, where $$d=4k+1$$ ($$k\in\mathbb{Z}$$) is a squarefree integer with $$d\neq1$$ (then $$\alpha$$ is an algebraic integer: $$\alpha^2+\alpha-k=0$$). The conjugation and norm are given by $$\overline{x+y\alpha}= x-y-y\alpha$$ and $$N(x+y\alpha)=x^2-xy-ky^2$$, where $$x,y\in\mathbb{Z}$$.

Some of these rings are Euclidean, such as $$\mathbb{Z}[\sqrt{d}]$$ for $$d=-2,-1,2,3,6,7$$ and $$\mathbb{Z}\left[\frac{-1+\sqrt{d}}2 \right]$$ for $$d=-7,-3,5$$.

Determining all quadratic unique factorization rings (including the non-Euclidean ones) is extremely serious. Among the rings of the type $$1^{\circ}$$ and $$2^{\circ}$$ with $$d < 0$$, apart from the ones mentioned already, the FTA holds in only five other rings: namely, the rings of the type $$2^{\circ}$$ for $$d=-11,-19,-43,-67,-163$$. Gauss’ conjecture that the FTA holds in infinitely many quadratic rings with a positive $$d$$ has not been proved nor disproved until today.

Problem 6 Find all integer solutions of the equation $$x^2+2=y^3$$.

Problem 7 Consider the sequence $$a_0,a_1,a_2,\dots$$ given by $$a_0=2$$ and $$a_{k+1}=2a_k^2-1$$ for $$k\geq0$$. Prove that if an odd prime number $$p$$ divides $$a_n$$, then $$p\equiv\pm1$$ (mod $$2^{n+2}$$).