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Problem :

Show the following inequality :

$$\int_{0}^{\infty}x^{-4\left(x^{2}-x-1\right)}dx> \int_{0}^{1}x^{-x}dx$$

My attempt :

I recall the equality :

$${\displaystyle {\begin{aligned}\int _{0}^{1}x^{-x}\,dx&=\sum _{n=1}^{\infty }n^{-n}\\\end{aligned}}}$$

Next I use a Reversed Cauchy Schwarz inequality see Reverse Cauchy Schwarz for integrals :

$$\frac{\sqrt{\int_{1}^{\frac{1+\sqrt{5}}{2}}x^{-8\left(x^{2}-x-1\right)}dx\cdot\int_{1}^{\frac{1+\sqrt{5}}{2}}1dx}}{\frac{1}{2}\left(\sqrt{\frac{f\left(1\right)}{f\left(1.315\right)}}+\sqrt{\frac{f\left(1.315\right)}{f\left(1\right)}}\right)}\leq\int_{1}^{\frac{1+\sqrt{5}}{2}}x^{-4\left(x^{2}-x-1\right)}dx$$

Wich is almost sufficient to show it but almost is not exactly .

Another path should be (sufficient to show inequality) :

$$\int_{\frac{10}{9}}^{\frac{1+\sqrt{5}}{2}}x^{-4\left(x^{2}-x-1\right)}dx-\exp\left(\int_{1}^{2}\ln\left(f\left(x\right)\right)dx\right)>0$$

Where :

$$f\left(x\right)=x^{4\left(-x^{2}+x+1\right)}$$

Here I'm stuck .

Brute forcing :

we have the following inequalities :

Let $1\leq x\leq 6/5$ then we have :

$$\frac{x^{4}\left(1-2\left(x^{2}-x\right)\left(x-1\right)\right)}{1+2\left(x^{2}-x\right)\left(x-1\right)}\leq x^{-4\left(x^{2}-x-1\right)}$$

Let $1/5\leq x\leq 1$ then (it seems) we have $a=-\frac{1}{\sqrt{3}}$ :

$$\frac{x^{4}\left(1-2\left(x^{2}-x+\frac{a}{2}\right)\left(x-1\right)\right)}{1+2\left(x^{2}-x-\frac{a}{2}\right)\left(x-1\right)}\leq x^{-4\left(x^{2}-x-1\right)}$$

It seems we have also for $3/2\leq x\leq 9/5$ and $b=2$:

$$f\left(x+\frac{77}{1000}\right)<x^{-4\left(x^{2}-x-1\right)}$$

Where ($b=2$) :

$$f\left(x\right)=\frac{9}{10}\left(x^{1-b}\left(1+\left(x-1\right)\left(x^{2}-b\right)+\frac{1}{2}\left(x-1\right)^{2}\left(\left(x^{2}-b\right)\left(x^{2}-1-b\right)\right)\right)^{-1}\left(1+\left(x-1\right)x+\left(x-1\right)^{2}x\left(x-1\right)\right)\right)^{4}$$

We are near from the true .

How to show it ?

Barackouda
  • 3,879
  • Also try to get the problem in the form of $\int_0^1 g(x)dx=\int_0^\infty x^{-4(x^2-x-1)}dx>\int_0^1 x^{-x}dx$. Then make the problem $g(x)>x^{-x}$ since both integrands are positive. However, there are other methods. – Тyma Gaidash Jul 24 '22 at 12:11
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    @TymaGaidash Thanks for your reply but such a simple path can be difficult no ? – Barackouda Jul 24 '22 at 12:22

0 Answers0