Number theoryDifficulty 5.8AIME, harderProve itCroatia
Determine all pairs (m,n) of integers such that n2−6n=m2+m−10.
Solution
We are given the equation n2−6n=m2+m−10. Let's rewrite it as n2−6n−m2−m+10=0. Group terms: n2−m2−6n−m+10=0. Recall that n2−m2=(n−m)(n+m), so (n−m)(n+m)−6n−m+10=0. Let us solve for n in terms of m.
Alternatively, move all terms to one side: n2−6n−m2−m+10=0 n2−6n=m2+m−10 This is a quadratic in n: n2−6n−(m2+m−10)=0 The discriminant must be a perfect square for integer solutions: Δ=36+4(m2+m−10)=36+4m2+4m−40=4m2+4m−4=4(m2+m−1) So 4(m2+m−1) must be a perfect square, say k2 for some integer k: 4(m2+m−1)=k2 So k2−4m2−4m+4=0 k2−4m2−4m+4=0 k2=4m2+4m−4 k2=4(m2+m−1) So m2+m−1 must be a perfect square times 1/4. But k must be even, so let k=2t for some integer t: (2t)2=4(m2+m−1) 4t2=4(m2+m−1) t2=m2+m−1 So m2+m−1=t2 for some integer t.
Now, rearrange: m2+m−1−t2=0 m2+m−t2=1 This is a quadratic in m: m2+m−(t2+1)=0 The discriminant must be a perfect square: Δ=12−4⋅1⋅(−t2−1)=1+4t2+4=4t2+5 So 4t2+5 must be a perfect square, say s2: s2=4t2+5 So s2−4t2=5
This is a Pell-type equation.
Let us solve s2−4t2=5 in integers.
Try small integer values for t: - t=0: s2=5 (no integer solution) - t=1: s2=4+5=9, s=3 - t=−1: s2=4+5=9, s=3 - t=2: s2=16+5=21 (no integer solution) - t=−2: s2=16+5=21 (no integer solution) - t=3: s2=36+5=41 (no integer solution) - t=−3: s2=36+5=41 (no integer solution) - t=5: s2=100+5=105 (no integer solution)
So only t=1 and t=−1 work, with s=3.
Now, recall m2+m−t2=1 For t=1: m2+m−1=1 m2+m=2 m2+m−2=0 m=2−1±1+8=2−1±3 So m=1 or m=−2
For t=−1: Same as above, m=1 or m=−2
Now, recall n is given by the quadratic: n2−6n=m2+m−10 For m=1: n2−6n=1+1−10=−8 n2−6n+8=0 n=26±36−32=26±2=4,2 For m=−2: n2−6n=4−2−10=−8 So same as above, n=4,2
Therefore, the integer solutions are: (m,n)=(1,2),(1,4),(−2,2),(−2,4)
Thus, all integer pairs (m,n) such that n2−6n=m2+m−10 are: (1,2),(1,4),(−2,2),(−2,4)
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