Algebraic number theory solves the ancient/long-standing problem of providing a proof of quadratic reciprocity that those of us who are not Gauss can actually remember. Let p be an odd prime, and let K be the field obtained from Q by adjoining a primitive pth root of 1. Then K contains a unique quadratic extension of Q, which one sees easily is that obtained by adjoining a square root of p or -p according as p is congruent to 1 mod 4 or not. Now let q be a second odd prime. By computing the action of the Frobenius at q on the unique quadratic subfield of K in two different ways, one obtains the main statement of quadratic reciprocity.
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