= Prescribing inert primes in a quadratic extension
At finitely many finite places of a <number field>, choose square-class parameters for the local unramified <quadratic extensions>. Local square classes are open, including at dyadic places by the <strong form of Hensel lemma>. <Weak approximation for number fields> gives a global parameter in all the prescribed local classes. Its quadratic extension is unramified of residue degree two at those places, so their extended prime ideals remain prime. If no finite place is prescribed, an auxiliary local nonsquare condition ensures the global extension is nontrivial.
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