Number theoryDifficulty 5.9AIME, harderFind the answer
Example 10 Find all pairs of positive integers (m,n) such that [n2]=[2+m2].
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Solution
Let (m,n) be a pair of positive integers satisfying (14), then n>m. Notice [(m+3)2]=[m2+32]⩾[m2]+[32]=[m2+4]
Therefore, n=m+1 or n=m+2. If n=m+1, since 21+2+21=1, by the Beatty-Rayleigh theorem, we know that {[2m]} and {[(2+2)h]} are complementary sequences, and at this time [n2]=[(m+1)2]=[2+m2]=2+[m2],
Thus, there is exactly one integer [(2+2)h] between [m2] and [(m+1)2], i.e., m2<(2+2)h<(m+1)2
Hence, m<(2+1)h<m+1, i.e., m=[(2+1)h]
Reversing the above process, we know that (m,n)=([(2+1)h],[(2+1)h]+1)
is a solution to (14). Therefore, the positive integer solutions to (14) in this case are
If n=m+2, similarly, we know [(m+2)2]=[2+m2]=2+[m2], so [m2],[(m+1)2], and [(m+2)2] are three consecutive positive integers. Therefore, there exists h∈N∗, such that [(2+2)(h+1)]−[(2+2)h]⩾4
However, [(2+2)(h+1)]−[(2+2)h]⩽1+[2+2]=4
Thus, there exists h∈N∗, such that {[(m−1)2]<[(2+2)h]<[m2][(2+2)(h+1)]=[(2+2)h]+4
From (15), we know m=[(1+2)h]+1, and (16) is [2(h+1)]=[2h+2]. Therefore, using the discussion of the first case (h is equivalent to m), we know there exists k∈N∗, such that h=[(1+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]
In the last step, we use m=[(1+2)[(1+2)k]]+1=[(3+22)k+1−(1+2)α]=[3k+2[2k]+1−(2−1)α]=3k+2[2k]
Here α={(1+2)k}={2k}.
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Source: NuminaMath-1.5,
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