# Computer software for periods

Kontsevich and Zagier define a period as an integral of a rational function (over $\mathbb{Q}$) defined on a $\mathbb{Q}$-semialgebraic set. They conjecture that if two periods are equal, then the reason may be only a finite number of change of variables and integrations by parts. The question is whether there is a clever computer system which checks whether two periods are equal and if they are, provide a proof (which is the aforementioned sequence.)

A particular motivation is the following identity \begin{align*} 7\int_0^1\log x\cdot \frac{1+x-x^2+x^3-x^4-x^5}{1-x^7}dx=\\ 12\int_{\frac{\sqrt{21}-5}2}^1 \frac{\log|x|}{2+3x+2x^2}dx. \end{align*}

Both parts are periods as may be easily seen if we expand $$\log |x|=\int_{|x|}^1 y^{-1}dy = p.v.\int_{x}^1 y^{-1} dy$$ and so get integrals of rational functions over triangles (if we are ok with p.v.) or triangle/quadrilateral.

Richard Stanley reminds us in this answer at MathOverflow that this is still a conjecture. It comes from the volumes of hyperbolic tetrahedra and it may be proved that the ratio of two sides is a rational number(!), which is calculated and coincides with 1 with accuracy of about 20000 decimal digits.

It is a bit annoying if neither a general theory nor programs motivated by this theory can provide a way to verify such a seemingly toy identity.

• is there a reason that one log has ||, while the other has not? – Dima Pasechnik Mar 11 '15 at 11:56
• @DimaPasechnik in LHS x is always positive. Or you mean philosophical reason? – Fedor Petrov Mar 11 '15 at 12:08
• oops, sorry. It's $\sqrt{21}-5$. I need better glasses. By the way: integrating polynomials over simplices is just differentiation (of the Laplace transform). One might be tempted to say that same holds for rational functions (which will need to be expanded...). I don't know if this is correct though (convergence is not obvious...). – Dima Pasechnik Mar 11 '15 at 12:12
• How about absolute convergence (a requirement in the definition of a period) of your double integrals on respective triangles? Is it obvious? – Dima Pasechnik Mar 11 '15 at 13:10
• In the LHS yes, in the RHS principal value becomes absolutely convergent after cancelling the sum $f(x,y)+f(x,-y)$ in small triangle corresponding to negative values of $x$. – Fedor Petrov Mar 11 '15 at 13:28

Mathematica 10.0.2.0 produces the LHS equals $$\frac{1}{7} \left(-\psi ^{(1)}\left(\frac{1}{7}\right)-\psi ^{(1)}\left(\frac{2}{7}\right)+\psi ^{(1)}\left(\frac{3}{7}\right)-\psi ^{(1)}\left(\frac{4}{7}\right)+\psi ^{(1)}\left(\frac{5}{7}\right)+\psi ^{(1)}\left(\frac{6}{7}\right)\right)$$ whereas the RHS is equal to $$-\frac{12 i \left(\text{Li}_2\left(-\frac{1}{4} i \left(-3 i+\sqrt{7}\right)\right)-\text{Li}_2\left(\frac{1}{4} i \left(3 i+\sqrt{7}\right)\right)+\text{Li}_2\left(\frac{2 i \left(-5+\sqrt{21}\right)}{-3 i+\sqrt{7}}\right)-\text{Li}_2\left(-\frac{2 i \left(-5+\sqrt{21}\right)}{3 i+\sqrt{7}}\right)+\log \left(\frac{1}{2} \left(5+\sqrt{21}\right)\right) \log \left(-7+7 i \sqrt{3}-5 i \sqrt{7}+3 \sqrt{21}\right)-\log \left(\frac{1}{2} \left(5+\sqrt{21}\right)\right) \log \left(-7-7 i \sqrt{3}+5 i \sqrt{7}+3 \sqrt{21}\right)\right)}{\sqrt{7}} .$$