Maths Olympiad Prep

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, 2014

Number theory Difficulty 5.6 AIME, harder Prove it Romania

Let ε\varepsilon be a positive real number. A positive integer will be called ε\varepsilon-squarish if it is the product of two integers aa and bb such that 1<a<b<(1+ε)a1 < a < b < (1 + \varepsilon)a. Prove that there are infinitely many occurrences of six consecutive ε\varepsilon-squarish integers.

Solution

If NN is a large enough positive integer, then N21=(N1)(N+1)N^2 - 1 = (N - 1)(N + 1) and N24=(N2)(N+2)N^2 - 4 = (N - 2)(N + 2) are both ε\varepsilon-squarish. Next, if kk is a large enough positive integer and N=(k1)(k+2)=k2+k2N = (k-1)(k+2) = k^2 + k - 2, then N2=(k1)2(k+2)2N^2 = (k-1)^2(k+2)^2, N22=(k22)(k2+2k1)N^2 - 2 = (k^2 - 2)(k^2 + 2k - 1) and N25=(k2k1)(k2+3k+1)N^2 - 5 = (k^2 - k - 1)(k^2 + 3k + 1) are all three ε\varepsilon-squarish. Finally, if nn is a large enough positive integer and N=2n22N = 2n^2 - 2, then N23=(2n22n1)(2n2+2n1)N^2 - 3 = (2n^2 - 2n - 1)(2n^2 + 2n - 1) is ε\varepsilon-squarish.

Consequently, N25N^2 - 5, N24N^2 - 4, \ldots, N2N^2 are six consecutive ε\varepsilon-squarish integers, provided that N=k2+k2=2n22N = k^2 + k - 2 = 2n^2 - 2, where kk and nn are sufficiently large integers. To conclude, write m=2k+1m = 2k + 1 to turn the condition into a Pell equation, m28n2=1m^2 - 8n^2 = 1, which has arbitrarily large solutions,
(mrnr)=(3813)r(10),rN. \begin{pmatrix} m_r \\ n_r \end{pmatrix} = \begin{pmatrix} 3 & 8 \\ 1 & 3 \end{pmatrix}^r \begin{pmatrix} 1 \\ 0 \end{pmatrix}, \quad r \in \mathbb{N}.

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