Diophantine equation

《初等数论》 · 第 75 页

变数个数多于方程个数，且变数取整数值的方程（或方程组）称为不定方程（或不定方程组）。

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《Introduction to Analytic Number Theory》 · 第 190 页 · Section 9.8「Applications to Diophantine equations」

Equations to be solved in integers are called Diophantine equations after Diophantus of Alexandria. An example is the equation $y^2=x^3+k$, where $k$ is a given integer.