Is there any explanation based on algebraic number theory that the integral $$ \int_{-\infty}^\infty\frac{dx}{\left(e^x+e^{-x}+e^{ix\sqrt{3}}\right)^2}=\frac{1}{3}\tag{1} $$ has a closed form? Analytic proof of this integral is given in this MSE post, however this proof does not explain why a similar looking integral $$ \int_{-\infty}^\infty\frac{e^{ix\sqrt{3}}\ dx}{\left(e^x+e^{-x}+e^{ix\sqrt{3}}\right)^3}=\frac{\sqrt{3}}{8\pi}\int_{0}^\infty\frac{dx}{\left(1+\frac{x^3}{1^3}\right)\left(1+\frac{x^3}{2^3}\right)\left(1+\frac{x^3}{3^3}\right)\ldots} $$ probably does not have a closed form. Is it possible that $(1)$ is related to Eisenstein integers?

Alternative formulation of the integral $(1)$ is $$ \int_0^\infty\frac{dt}{(1+t+t^{\,\alpha})^2}=\frac23, \quad \alpha=\frac{1+i\sqrt3}{2}. \tag{1a} $$

  • $\begingroup$ Why do you think there is an explanation based on algebraic number theory? You could equally ask for an explanation based on algebraic topology or probability or combinatorics or what not. $\endgroup$ – GH from MO Mar 2 '17 at 13:06
  • 6
    $\begingroup$ @GHfromMO , it seems that there might be connection with Eisenstein integers, but I wouldn't be too surprised if this was wrong. I also couldn't find a parametric generalization of (1), although there are other integrals of this kind with $(e^x+e^{-x}+e^{ix\sqrt{3}})^{a+3n}$, where $a=2,1/2$ and $n\ge 0$ an integer, in the denominator. There is some possibility that an isolated integral evaluation has relation to algebraic number theory. Only one proof of (1) is known so far, so any other proofs are welcomed, because they might show how to generalize (1) or explain why it is isolated result. $\endgroup$ – user82588 Mar 2 '17 at 14:13
  • $\begingroup$ For example the Herglotz integral is from the category of integrals related to number theory math.stackexchange.com/questions/522913/… $\endgroup$ – user82588 Mar 2 '17 at 14:16

The following formula gives a parametric extension of $(1)$ for $|a|$ sufficiently small \begin{align} \small\int_{-\infty}^\infty\frac{dx}{\left(e^x+e^{-x}+e^{a+ix\sqrt{3}}\right)^2}+e^a\int_{-\infty}^\infty\frac{dx}{\left(e^{a+x}+e^{-x}+e^{ix\sqrt{3}}\right)^2}+e^a\int_{-\infty}^\infty\frac{dx}{\left(e^{a+x}+e^{-x}+e^{-ix\sqrt{3}}\right)^2}=1 \end{align} This means that $(1)$ is not an isolated result. In view of this one might be very sceptical that any number theoretic interpretation of the integral exists.


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.