Solution
ID: past-exam-of-the-mathematics-course-of-the-university-of-cambridge/2023/ii/paper-1/1g/iii/solution
Past exam of the mathematics course of the University of Cambridge 2023 ii Paper 1 1G iii Solution by
Codex 0 Created 2026-09-24 Updated 2026-09-25
The quadratic form factors as a sum of two squares:The change of variableshas determinant , so its matrix is a unimodular matrix and it maps bijectively to itself. The original equation is therefore equivalent toThe sum of two squares theorem also decides this systematically: , and every prime factor congruent to modulo occurs to an even power, vacuously here. Thus is a sum of two integer squares. Explicitly,Inverting the change of variables gives, for example, . ConsequentlyThis is the unimodular reduction of a discriminant-minus-four form to two squares.
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