A **Diophantine equation** (named after Diophantos of Alexandria) is an equation with one or more integer variables $x,y,z,\ldots$ (often involving also their powers) and with integer coefficients $a,b,c,\ldots$

A Diophantine equation is called **linear** (**quadratic**, **cubic**, etc. or, in general, **$n$th order**) if all variables have at most the power of $1$ ($2$, $3$, etc. $n$).

- $2{x} +5y=16$ (linear),
- $-x_1+2x_2+3x_3-3x_4=0$ (linear),
- $x^2+y^2=z^2$ (quadratic),
- $2xy+y^2-z^3=1$ (cubic).
- for $x^n+yz=z^2$ ($n$th order).

It is convenient to write a given Diophantine equation in the form $f(x_1,\ldots,x_r)=0$ where $x_1,\ldots,x_r$ are the variables of this equation. The above equation defines interpreted as a function $f:\mathbb Z^r\to \mathbb Z.$ The above examples can be re-written as

- $2{x} +5y-16=0$,
- $-x_1+2x_2+3x_3-3x_4=0$ (already in this form),
- $x^2+y^2-z^2=0$,
- $x^4+y^4-z^2=0$,
- $2xy+y^2-z^3-1=0$
- $x^n+yz-z^2=0.$

| | | | | created: 2019-04-13 09:09:12 | modified: 2019-06-20 17:09:37 | by: *bookofproofs* | references: [1272], [8152]

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[8152] **Jones G., Jones M.**: “Elementary Number Theory (Undergraduate Series)”, Springer, 1998

[1272] **Landau, Edmund**: “Vorlesungen über Zahlentheorie, Aus der Elementaren Zahlentheorie”, S. Hirzel, Leipzig, 1927