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Number theory Difficulty 5.9 AIME, harder Find the answer

Example 10 Find all pairs of positive integers (m,n)(m, n) such that
[n2]=[2+m2].[n \sqrt{2}]=[2+m \sqrt{2}] .

A number or a short expression. Spacing and $ signs are ignored.

Solution

Let (m,n)(m, n) be a pair of positive integers satisfying (14), then n>mn>m.
Notice
[(m+3)2]=[m2+32][m2]+[32]=[m2+4]\begin{aligned} {[(m+3) \sqrt{2}] } & =[m \sqrt{2}+3 \sqrt{2}] \geqslant[m \sqrt{2}]+[3 \sqrt{2}] \\ & =[m \sqrt{2}+4] \end{aligned}

Therefore, n=m+1n=m+1 or n=m+2n=m+2.
If n=m+1n=m+1, since 12+12+2=1\frac{1}{\sqrt{2}}+\frac{1}{2+\sqrt{2}}=1, by the Beatty-Rayleigh theorem, we know that {[2m]}\{[\sqrt{2} m]\} and {[(2+2)h]}\{[(2+\sqrt{2}) h]\} are complementary sequences, and at this time
[n2]=[(m+1)2]=[2+m2]=2+[m2],[n \sqrt{2}]=[(m+1) \sqrt{2}]=[2+m \sqrt{2}]=2+[m \sqrt{2}],

Thus, there is exactly one integer [(2+2)h][(2+\sqrt{2}) h] between [m2][m \sqrt{2}] and [(m+1)2][(m+1) \sqrt{2}], i.e.,
m2<(2+2)h<(m+1)2m \sqrt{2}<(2+\sqrt{2}) h<(m+1) \sqrt{2}

Hence, m<(2+1)h<m+1m<(\sqrt{2}+1) h<m+1, i.e.,
m=[(2+1)h]m=[(\sqrt{2}+1) h]

Reversing the above process, we know that
(m,n)=([(2+1)h],[(2+1)h]+1)(m, n)=([(\sqrt{2}+1) h],[(\sqrt{2}+1) h]+1)

is a solution to (14). Therefore, the positive integer solutions to (14) in this case are

If n=m+2n=m+2, similarly, we know [(m+2)2]=[2+m2]=2+[m2][(m+2) \sqrt{2}]=[2+m \sqrt{2}]=2+[m \sqrt{2}], so [m2],[(m+1)2][m \sqrt{2}],[(m+1) \sqrt{2}], and [(m+2)2][(m+2) \sqrt{2}] are three consecutive positive integers. Therefore, there exists hNh \in \mathbf{N}^{*}, such that
[(2+2)(h+1)][(2+2)h]4[(2+\sqrt{2})(h+1)]-[(2+\sqrt{2}) h] \geqslant 4

However,
[(2+2)(h+1)][(2+2)h]1+[2+2]=4[(2+\sqrt{2})(h+1)]-[(2+\sqrt{2}) h] \leqslant 1+[2+\sqrt{2}]=4

Thus, there exists hNh \in \mathbf{N}^{*}, such that
{[(m1)2]<[(2+2)h]<[m2][(2+2)(h+1)]=[(2+2)h]+4\left\{\begin{array}{l} {[(m-1) \sqrt{2}]<[(2+\sqrt{2}) h]<[m \sqrt{2}]} \\ {[(2+\sqrt{2})(h+1)]=[(2+\sqrt{2}) h]+4} \end{array}\right.

From (15), we know m=[(1+2)h]+1m=[(1+\sqrt{2}) h]+1, and (16) is [2(h+1)]=[2h+2][\sqrt{2}(h+1)]=[\sqrt{2} h+2]. Therefore, using the discussion of the first case (hh is equivalent to mm), we know there exists kNk \in \mathbf{N}^{*}, such that h=[(1+2)k]h=[(1+\sqrt{2}) k]. Reversing the above process, we know that the positive integer solutions to (14) in this case are
{m=[(1+2)h]+1=3k+2[2k],n=3k+2+2[2k]\left\{\begin{array}{l} m=[(1+\sqrt{2}) h]+1=3 k+2[\sqrt{2} k], \\ n=3 k+2+2[\sqrt{2} k] \end{array}\right.

In the last step, we use
m=[(1+2)[(1+2)k]]+1=[(3+22)k+1(1+2)α]=[3k+2[2k]+1(21)α]=3k+2[2k]\begin{aligned} m & =[(1+\sqrt{2})[(1+\sqrt{2}) k]]+1 \\ & =[(3+2 \sqrt{2}) k+1-(1+\sqrt{2}) \alpha] \\ & =[3 k+2[\sqrt{2} k]+1-(\sqrt{2}-1) \alpha] \\ & =3 k+2[\sqrt{2} k] \end{aligned}

Here α={(1+2)k}={2k}\alpha=\{(1+\sqrt{2}) k\}=\{\sqrt{2} k\}.

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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.