Lemma 27.3.4. A scheme $X$ is integral if and only if it is reduced and irreducible.

**Proof.**
If $X$ is irreducible, then every affine open $\mathop{\mathrm{Spec}}(R) = U \subset X$ is irreducible. If $X$ is reduced, then $R$ is reduced, by Lemma 27.3.2 above. Hence $R$ is reduced and $(0)$ is a prime ideal, i.e., $R$ is an integral domain.

If $X$ is integral, then for every nonempty affine open $\mathop{\mathrm{Spec}}(R) = U \subset X$ the ring $R$ is reduced and hence $X$ is reduced by Lemma 27.3.2. Moreover, every nonempty affine open is irreducible. Hence $X$ is irreducible, see Lemma 27.3.3. $\square$

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