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Given two natural numbers $p$ and $i$, such that $0 < i \leqslant 2^p$, let $$ \Phi(p,i) := \frac{1}{2^p+1} + \frac{1}{(i+1)^2} - \frac{1}{2^p}\lg\left(\frac{2^p}{i}+1\right), $$ where $\lg x$ is the binary logarithm. With the help of a Computer Algebra System, it seems that

  • If $0 \leqslant p \leqslant 3$, then $\Phi(p,i) < 0$.

  • If $4 \leqslant p$, there exists $i_p$ such that $\Phi(p,i_p) = 0$ and $\Phi(p,i) > 0$ for $1 \leqslant i < i_p$, and $\Phi(p,i) < 0$ for $i_p < i \leqslant 2^p$.

How can I prove this?

Just in case, the partial derivative with respect to $i$ is: $$ \frac{\partial\Phi}{\partial i}(p,i) = \frac{1}{i(2^p+i)\ln 2} - \frac{2}{(i+1)^3}, $$ where $\ln x$ is the natural logarithm.

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1 Answer

up vote 0 down vote accepted

Peter Mueller at MathOverflow solved the problem: http://mathoverflow.net/questions/108816/root-and-sign-of-a-complicated-bivariate-function

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