**Definition**: Quadratic Residue, Quadratic Nonresidue

Let $m > 0$ be a positive integer. An integer $n\in\mathbb Z$ is called a **quadratic residue modulo** $m,$ if the congruence $$x^2(m)\equiv n(m)$$ is solvable, otherwise it is called a **quadratic nonresidue**.

### Examples

- For the module $m=1$, any integers $n$ is a residue, since $x^2(1)\equiv 0 (1)\equiv n(1)$ is always solvable.
- If $n=0,$ $n=1,$ or $n$ is a perfect square, then $n$ is a quadratic residue modulo any positive integer $m > 0.$

| | | | | created: 2019-05-12 08:45:49 | modified: 2019-05-12 09:34:21 | by: *bookofproofs* | references: [1272]

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

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