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Number theory Difficulty 5.8 AIME, harder Prove it Saudi Arabia

Find the smallest prime qq such that
q=a12+b12=a22+2b22=a32+3b32==a102+10b102 q = a_{1}^{2} + b_{1}^{2} = a_{2}^{2} + 2 b_{2}^{2} = a_{3}^{2} + 3 b_{3}^{2} = \ldots = a_{10}^{2} + 10 b_{10}^{2}
where ai,bia_{i}, b_{i} (i=1,2,,10)(i=1,2, \ldots, 10) are positive integers.

Solution

Since q=a12+b12q = a_{1}^{2} + b_{1}^{2} then qq must have the form 4k+14k+1; otherwise, a12+b12a_{1}^{2} + b_{1}^{2} is divisible by q=4k+3q = 4k+3 which implies that qa1,qb1q \mid a_{1}, q \mid b_{1}, a contradiction.

Note that q=a22+2b22q = a_{2}^{2} + 2 b_{2}^{2} so in modulo 88, number qq is congruent to either 11 or 33 modulo 88, but q1(mod4)q \equiv 1 \pmod{4} then q1(mod8)q \equiv 1 \pmod{8}.

Continue, q=a32+3b32q = a_{3}^{2} + 3 b_{3}^{2} so by taking modulo 33, it gives qq is congruent to 11 in modulo 33. Similarly, by considering two equations q=a52+5b52q = a_{5}^{2} + 5 b_{5}^{2} and q=a72+7b72q = a_{7}^{2} + 7 b_{7}^{2}, we have q1,4(mod5)q \equiv 1,4 \pmod{5} and q1,2,4(mod7)q \equiv 1,2,4 \pmod{7}.

These congruences can be put together as qq is congruent to
1,121,169,289,361,529 in modulo 840. 1, 121, 169, 289, 361, 529 \text{ in modulo } 840.
The first smallest values that satisfy the previous congruence are
1,121,361,169,289,529,841,961,1009, 1, 121, 361, 169, 289, 529, 841, 961, 1009, \ldots
and it is easy to check that the smallest prime solution is 10091009.

And it does satisfy the problem requirement by the following equalities
1009=282+152=192+2(182)=312+3(42)=152+4(192)=172+5(122)=252+6(82)=12+7(122)=192+8(92)=282+9(52)=32+10(102) \begin{aligned} & 1009 \\ & = 28^{2} + 15^{2} = 19^{2} + 2(18^{2}) \\ & = 31^{2} + 3(4^{2}) = 15^{2} + 4(19^{2}) \\ & = 17^{2} + 5(12^{2}) = 25^{2} + 6(8^{2}) \\ & = 1^{2} + 7(12^{2}) = 19^{2} + 8(9^{2}) \\ & = 28^{2} + 9(5^{2}) = 3^{2} + 10(10^{2}) \end{aligned}
Therefore, the answer is 10091009.

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Source: MathNet, licensed CC-BY-4.0. Statement and solution reproduced as published; topic and difficulty added by this site.