Maths Olympiad Prep

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Number theory Difficulty 6.4 National olympiad Prove it

14. Prove: The indeterminate equation x2=y54x^{2}=y^{5}-4 has no integer solutions.

Solution

14. Suppose the original equation has integer solutions (x,y)(x, y). If 11y11 \mid y, then x27(mod11)x^{2} \equiv 7(\bmod 11); if 11y11 \nmid y, then by Fermat's Little Theorem, we know 11(y101)11 \mid\left(y^{10}-1\right), i.e., 11(y51)(y5+1)11 \mid\left(y^{5}-1\right)\left(y^{5}+1\right). Since (y51,y5+1)2\left(y^{5}-1, y^{5}+1\right) \mid 2, it follows that y5±1(mod11)y^{5} \equiv \pm 1(\bmod 11), hence x26x^{2} \equiv 6 or 8(mod11)8(\bmod 11). Therefore, we need x26,7x^{2} \equiv 6, 7 or 8(mod11)8(\bmod 11). However, by discussing x0,±1,±2,±3,±4,±5x \equiv 0, \pm 1, \pm 2, \pm 3, \pm 4, \pm 5 (mod11)(\bmod 11), we find that x20,1,4,9,5x^{2} \equiv 0, 1, 4, 9, 5 or 3(mod11)3(\bmod 11), and it is impossible for x26x^{2} \equiv 6, 7 or 8(mod11)8(\bmod 11), leading to a contradiction.

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Source: NuminaMath-1.5, licensed Apache-2.0. Statement and solution reproduced as published; topic and difficulty added by this site.