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Given the identities:

  1. $3^p - 2^q = 3^p (1 - 3^{q\left(\frac{\log{2}}{\log{3}} - \frac{p}{q}\right)})$ for all integers $p, q$ with $q$ positive.

  2. $\left|3^p - 2^q\right| \ge 1$ for all positive integers $p, q$.

Show there exists a constant $c > 0$, such that

$\left|\frac{\log{2}}{\log{3}} - \frac{p}{q}\right| \ge c \frac{1}{2^q}$

for all integers $p, q$ where $q > 0$.

(Paraphrased from Proposition 5 (Trivial bound) in Terence Tao's blog article.)

I assumed it would just involve some algebra, but I couldn't solve it after scribbling a page full of notes.

  • You can get properly sized parentheses (and other paired delimiters) that adapt to their content by preceding them with \left and \right. – joriki Jan 17 '23 at 08:09
  • Changed based on suggestion, but I don't really like how the bar is now broken in the middle. – spenceryue Jan 17 '23 at 23:06
  • Interesting – I don't get a bar broken in the middle here – it looks great now in my browser. Which browser are you using? Anyway, what I mostly meant were the awful tiny parentheses in identity 1. – joriki Jan 17 '23 at 23:21
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    I can see how to to obtain the weaker bound $ \frac{c}{q 2^q}$---via a combination of case work and Taylor expansion. Presumably some other insight is needed to remove the extra poly factor. – Drew Brady Jan 18 '23 at 01:09
  • @joriki, I'm using Chrome 109.0.5414.87 (arm64) on macOS 13.1. (Updated identity 1 formatting.) – spenceryue Jan 21 '23 at 04:06

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