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Number theory Difficulty 5.5 AIME, harder Prove it Greece

Determine the natural numbers vv for which 2007+4v2007 + 4v is perfect square.

Solution

Let 2007+4v=κ22007 + 4v = \kappa^2, κN\kappa \in \mathbb{N}. Then
κ24v=2007κ222v=2007(κ2v)(κ+2v)=133223. \kappa^2 - 4v = 2007 \Leftrightarrow \kappa^2 - 2^{2v} = 2007 \Leftrightarrow (\kappa - 2^v)(\kappa + 2^v) = 1 \cdot 3 \cdot 3 \cdot 223.
Since κ2v<κ+2v\kappa - 2^v < \kappa + 2^v, last equality is equivalent to the systems
{κ2v=1κ+2v=2007 or {κ2v=3κ+2v=669 or {κ2v=9κ+2v=223(Σ1) or (Σ2) or (Σ3) \begin{cases} \kappa - 2^v = 1 \\ \kappa + 2^v = 2007 \end{cases} \text{ or } \begin{cases} \kappa - 2^v = 3 \\ \kappa + 2^v = 669 \end{cases} \text{ or } \begin{cases} \kappa - 2^v = 9 \\ \kappa + 2^v = 223 \end{cases} \quad (\Sigma_1) \text{ or } (\Sigma_2) \text{ or } (\Sigma_3)
Now we have
(Σ1){κ2v=1κ+2v=2007{2κ=200822v=20062v=1003, absurd.. (\Sigma_1) \Leftrightarrow \begin{cases} \kappa - 2^v = 1 \\ \kappa + 2^v = 2007 \end{cases} \Leftrightarrow \begin{cases} 2\kappa = 2008 \\ 2 \cdot 2^v = 2006 \end{cases} \Rightarrow 2^v = 1003, \text{ absurd..}
(Σ2){κ2v=3κ+2v=669{2κ=67222v=6662v=333, absurd. (\Sigma_2) \Leftrightarrow \begin{cases} \kappa - 2^v = 3 \\ \kappa + 2^v = 669 \end{cases} \Leftrightarrow \begin{cases} 2\kappa = 672 \\ 2 \cdot 2^v = 666 \end{cases} \Rightarrow 2^v = 333, \text{ absurd.}
(Σ3){κ2v=9κ+2v=223{2κ=23222v=2142v=107, absurd. (\Sigma_3) \Leftrightarrow \begin{cases} \kappa - 2^v = 9 \\ \kappa + 2^v = 223 \end{cases} \Leftrightarrow \begin{cases} 2\kappa = 232 \\ 2 \cdot 2^v = 214 \end{cases} \Rightarrow 2^v = 107, \text{ absurd.}
Therefore, there are not natural numbers vv such that 2007+4v2007 + 4v is a perfect square.

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