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When playing around with modular form integrations, one may accidentally come across the following mysterious integral:

\begin{align*} I=\int_0^{i\infty}\frac{E_4(z)}{\sqrt{j(z)}}dz\approx 0.0368929462609\sqrt{-1}. \end{align*}

Here $E_4(z)=1+240q+O(q^2)$ is the modular Eisenstein series of $\Gamma=\Gamma(1)=\mathrm{SL}(2,\mathbb Z)$ of weight 4, and $j(z)=\frac 1q+744+O(q)$ is the $j$-invariant. The interesting stuff is that by numerically check:

\begin{align*} -\frac16\frac{L(f_\mathsf{24.4.a.a},1)}{2\pi i}\approx 0.0368929462609\sqrt{-1}. \end{align*}

So my question is, can one prove (or disprove) the following?

$$I\stackrel{?}{=}-\frac16\frac{L(f_\mathsf{24.4.a.a},1)}{2\pi i}. $$


Through a series of transformations and computations (the process is too long to show), by using the Hauptmodul of the modular group $\Gamma_0(6)$, I transformed the problem into the following form:

\begin{align*} &2\int_0^1 {_2F_1}\left(\frac13,\frac23,1;\lambda\right)^2\cdot\sqrt{\frac{1-\lambda}{3\lambda(1+8\lambda)}}d\lambda\\ \stackrel ?=&\int_0^1 {_2F_1}\left(\frac13,\frac23,1;\frac{27t^2}{(4-t)^3}\right)^2\cdot\frac{8+t^2}{(4-t)^2}\sqrt{\frac{1}{(1-t)t(8+t)}}dt. \end{align*}

So it would also be great if a proof of the above integral expression could be obtained. (If everything is right, now both sides should be equal to $L(f_\mathsf{24.4.a.a},1)\approx 1.39083130731041173$)


The periods of cusp form and "modular form"

It turns out that the $L$-function result is a period of a level 24 weight 4 modular form (see this LMFDB link, it is also the unique modular form of level 24 weight 4 and with trivial character) which has $q$-series expansion: \begin{align*} f_\mathsf{24.4.a.a}(q)= q + 3q^3 + 14q^5 - 24q^7 + 9q^9 - 28q^{11} +O\left(q^{13}\right) \end{align*}

This is intriguting in two aspects. Firstly the level 24 seems to be coming from nowhere since our original interests are $E_4$ and $j$, which are level 1. Secondly the integrand $E_4/\sqrt{j}$ is not cuspidal, actually not holomorphic, and it even has branch points on the upper half plane $\mathfrak H$. Luckily one can still compute the $q$-expansion at $z=i\infty$ and get a power series of $q$ multiplied by a $q^{1/2}$ term which looks like:

\begin{align*} \frac{E_4(q)}{\sqrt{j(q)}}=q^{1/2}\left(1 - 132 q + 22014 q^2 - 4187368 q^3 + 841843701 q^4+O\left(q^5\right)\right). \end{align*}

So it is at least not obvious that what is the relation of $E_4/\sqrt j$ and $f_\mathsf{24.4.a.a}$.


Lifting the modular level

I'm also trying to lift the level of $E_4/\sqrt j$ and here is some progress:

Notice that $j=E_4^3/\Delta$ so $E_4=(j\Delta)^{1/3}$, hence we get

\begin{align*} \frac{E_4}{\sqrt j}=\frac{(j\Delta)^{1/3}}{j^{1/2}}=\frac{\Delta^{1/3}}{j^{1/6}}. \end{align*}

At first glance this is pretty much useless since we are now facing exponent $1/3,1/6$. However on one hand we have the famous identity $\Delta=\eta^{24}$ where $\eta$ is the Dedekind $\eta$-function. So the numerator $\Delta^{1/3}=\eta^8$. This reminds me of another MSE problem (this MSE link) which mainly concerns

\begin{align*} I(n):=\int_{0}^\infty\eta(ix)^n dx. \end{align*}

Luckily by that problem $I(8)=\frac{\sqrt 3}{(2\pi)^6}\Gamma(\frac 13)^{9}$. In fact we have \begin{align*} I(8)=3i\cdot\frac{L(f_\mathsf{9.4.a.a},1)}{2\pi i}. \end{align*} This is because $f_\mathsf{9.4.a.a}=\eta(3\tau)^8$ and it has CM.


The Hauptmodul $j^{1/3}$

We may also consider $j^{1/3}$, which is: \begin{align*} j^{1/3}(q^3)=\frac 1q + 248 q^2 + 4124 q^5 + 34752 q^8 + 213126 q^{11} + 1057504 q^{14}+O\left( q^{17}\right). \end{align*}

It is known from OEIS (OEIS 7245) that the sequence is actually the McKay-Thompson series of class 3C, that is $T_\mathsf{3C}(q)$. So it is a Hauptmodul of the genus $0$ modular group $\Gamma(3)$. In other words, this is a special generator of the modular function field of $\Gamma(3)$ with $q$-expansion $q^{-1}+O(q)$.

All these observations tell us that we have lifted the level to 3 or 9 ($\Gamma(3)$ or $\Gamma_0(9)$). Maybe one can go further by considering more Hauptmoduln.

From other Hauptmoduln we get also

\begin{align*} \frac{\sqrt{-1}}{2\pi}\int_0^1{_2F_1}\left(\frac 13,\frac 23,1;\lambda\right)^2\sqrt{\frac{1-\lambda}{27\lambda(1+8\lambda)}}d\lambda\approx 0.0368929462609\sqrt{-1}, \\ \frac{\sqrt{-1}}{16\pi}\int_0^1{_2F_1}\left(\frac 12,\frac 12,1;\lambda\right)^2\sqrt{\frac{1}{1-\lambda+\lambda^2}}d\lambda\approx 0.0368929462609\sqrt{-1}. \end{align*}

These identities include modular reparametrizations of the $(E_4/\sqrt j)dz$. One thing which might be helpful is that:

$$y^2=\frac{1-x}{27x(1+8x)}$$

defines an elliptic curve with $j=1556068/81$, which has LMFDB label $\mathsf{24.a3}$. And the modular curve is $X_0(24)$, that is $\mathsf{24.a4}$. So by checking on the LMFDB database, one finds a degree two isogeny between them.


Can anyone help me about it? Thank you very much!

cybcat
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  • there used to be a nice answer to this question, where did it go? – Roccooi Sep 04 '25 at 10:19
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    This integral is mentioned in PhD thesis of K. Bönisch, page 84, where a relation between $$\int_0^{i\infty} \frac{E_4}{\sqrt{j}}\tau d\tau$$ and $L(f,2)$ is also mentioned. – pisco Sep 04 '25 at 14:52

1 Answers1

11

As noted in the comments, identities like this have been mentioned in Modularity of special motives of rank four associated with Calabi-Yau threefolds. Below I sketch the proof of a statement that is slightly more general than yours, namely that for any contour $\gamma$ on $\overline{\mathbb{H}} \setminus j^{-1}(0)$ from one cusp to another one, there are $r^\pm(\gamma) \in \mathbb{Z}^3$ (which we construct explicitly) such that \begin{align*} \int_{\gamma} \, \begin{pmatrix} 1 \\ \tau \\ \tau^2 \end{pmatrix} \frac{E_4(\tau)}{\sqrt{j(\tau)}} \, \mathrm{d} \tau \, = \, \omega^+ \, r^+(\gamma) + \omega^- \, r^-(\gamma) \, . \end{align*} Here, the integrand is understood to be analytically continued along the contour and the numbers $\omega^\pm$ are periods of the unique newform $f \in S_4(\Gamma_0(24))$. In terms of $L$-function values the periods $\omega^\pm$ can be expressed as \begin{align*} \omega^+ \, &= \, - \frac{L(f,2)}{(2\pi i)^2} \, = \, 0.0341294416212 \cdots \\ \omega^- \, &= \, -\frac{1}{6} \frac{L(f,1)}{2\pi i} \, = \, 0.0368929462609 \cdots i \, . \end{align*}

General idea

We consider the hypergeometric function \begin{align*} \Pi(z) \, = \, _4F_3\left(\frac{1}{2}, \frac{1}{2}, \frac{1}{6}, \frac{5}{6}; 1, 1, 1; z\right) \end{align*} for $0 < z < 1$ and the elementary identities \begin{align*} \Pi(z) \, = \, \frac{1}{2\pi i} \oint \frac{1}{\sqrt{1-t}} \, _3F_2\left(\frac{1}{2}, \frac{1}{6}, \frac{5}{6}; 1, 1; \frac{z}{t}\right) \, \frac{\mathrm{d}t}{t} \, =: \, \oint A_{z, 1}(t) \end{align*} and \begin{align*} \Pi(z) \, = \, \frac{1}{2\pi i} \oint \frac{1}{\sqrt{1-t}} \, _3F_2\left(\frac{1}{4}, \frac{1}{2}, \frac{3}{4}; 1, 1; \frac{2^2}{3^3} \frac{z}{t \, (1-t)^2}\right) \, \frac{\mathrm{d}t}{t} \, =: \, \oint B_{z, 1}(t) \end{align*} for suitable contours (e.g. circles counterclockwise around $0$ with fixed radii $|z| < |t| < 1$ for the first and $|t| = 1/3$ for the second). After acting by a monodromy, taking the limit $z \rightarrow 1$ and pulling back by certain modular functions, the first identity will lead to integrals of $E_4/\sqrt{j}$ while the second identity will lead to integrals of the newform $f$.

Because $\Pi$ has the undesired property that it is not holomorphic around $z = 1$, we will consider the linear combination \begin{align*} \widetilde{\Pi} \, = \, 4 \, \Pi + M_0 M_1 \Pi \, , \end{align*} where $M_0$ and $M_1$ correspond to the analytic continuation along loops counterclockwise around $0$ and $1$, respectively.

Integrals of $A$

To obtain an integral expression for $\widetilde{\Pi}$, first note that $A_{z, 1}$ is part of a local system of rank three on $\mathcal{M}_z \, = \, \mathbb{P}^1 \setminus \{0,z,1,\infty\}$. We fix some $t_0 > 1$ and choose the following generators of $\pi_1(\mathcal{M}_z, t_0)$:

Generators of fundamental group

For the local system we can choose a vector of basis elements $A_z$ around $t_0$ such that the first component is $A_{z, 1}$ (with the branch of the square root chosen such that $1/\sqrt{1-t_0} \in i \mathbb{R}_{>0}$) and the monodromy matrices (acting by $A_z \mapsto M_{\gamma} \, A_z$) are given by $M_{\gamma_1} = -T, M_{\gamma_2} = -1, M_{\gamma_3} = S$ in terms of \begin{align*} T \, = \, \begin{pmatrix} 1 & 0 & 0 \\ 1 & 1 & 0 \\ 1 & 2 & 1 \end{pmatrix} \qquad \text{and} \qquad S \, = \, \begin{pmatrix} 0 & 0 & 1 \\ 0 & -1 & 0 \\ 1 & 0 & 0 \end{pmatrix} \, . \end{align*} Now consider the map \begin{align*} r_z : \ \pi_1(\mathcal{M}_z, t_0) \, &\rightarrow \, \mathbb{C}^3 \\ \gamma \, &\mapsto \, \int_{\gamma} A_z \, . \end{align*} It is easy to see that this satisfies the cocycle property $r_z(\gamma \gamma') = r(\gamma) + M_{\gamma} \, r(\gamma')$ and hence it is also completely determined by its values on a set of generators. The analytic continuations in $z$ give the following actions: \begin{align*} \begin{array}[h]{c|ccc} \gamma & \gamma_1 & \gamma_2 & \gamma_3 \\ \hline M_0 \, r_z(\gamma) & T \, r_z(\gamma_1) & T \, r_z(\gamma_2) & T \, r_z(\gamma_1^{-1}\gamma_2^{-1}\gamma_3\gamma_1\gamma_2^{-1}) \\ M_1 \, r_z(\gamma) & r_z(\gamma_1) & r_z(\gamma_3\gamma_2\gamma_3^{-1}) & r_z(\gamma_3) \end{array} \end{align*} From $\Pi(z) = (1, 0, 0) \cdot r_z(\gamma_1^{-1}\gamma_2^{-1})$ and the analytic continuation we then get \begin{align*} \widetilde{\Pi}(z) \, &= \, \begin{pmatrix} 3 \\ 0 \\ -2 \end{pmatrix} \cdot r_z(\gamma_1) + \begin{pmatrix} -1 \\ 2 \\ 1 \end{pmatrix} \cdot r_z(\gamma_2) + \begin{pmatrix} -2 \\ 0 \\ 0 \end{pmatrix} \cdot r_z(\gamma_3) \\ &= \, \begin{pmatrix} 3 \\ 0 \\ -2 \end{pmatrix} \cdot r_z(\gamma_1) + \begin{pmatrix} -3 \\ 2 \\ 1 \end{pmatrix} \cdot r_z(\gamma_3) \, . \end{align*} Here, we used that $(-1,2,1) \cdot r_z(\gamma_2) = (-1,2,1) \cdot r_z(\gamma_3)$ since the integrand is holomorphic around $t = z$ and hence the countour $\gamma_2$ can be pushed outside of the region that gets pinched for $z \rightarrow 1$.

We now restrict to $z = 1$. Then $r_1$ is completely determined by the values on the two generators $\gamma_1$ and $\gamma_3$. Further, we know that $(1,0,1) \cdot r_1(\gamma_3) = 0$ since the integrand is holomorphic around $t = 1$ and hence \begin{align*} r_1(\gamma_3) \, = \,\lambda^+ \, \begin{pmatrix} -1 \\ 0 \\ 1 \end{pmatrix} + \lambda^- \, \begin{pmatrix} 0 \\ 1 \\ 0 \end{pmatrix} \end{align*} for some $\lambda^\pm \in \mathbb{C}$. It is easy to see that in fact $\lambda^+$ must be real while $\lambda^-$ must be purely imaginary. In the limit $t_0 \rightarrow \infty$ these numbers completely determine $r_1$ since then $r_1(\gamma_1) = 0$. In particular, we can then read off $\lambda^\pm$ from \begin{align*} \widetilde{\Pi}(1) \, = \, 4 \, \lambda^+ + 2 \, \lambda^- \, . \end{align*} As a last step, we consider the modularity of $A$. Pulling back by $t_1 = \frac{1728}{j}$ we get \begin{align*} (t_1^*A)(\tau) \, = \, -24\sqrt{3}i \, \begin{pmatrix} 1 \\ \tau \\ \tau^2 \end{pmatrix} \frac{E_4(\tau)}{\sqrt{j(\tau)}} \, \mathrm{d}\tau \end{align*} around $\tau = \infty$. Every contour $\gamma$ on $\overline{\mathbb{H}} \setminus j^{-1}(0)$ from $\infty$ to a cusp corresponds to an element of $\pi_1(\mathcal{M}_z, t_0)$ (in the limit $t_0 \rightarrow \infty$) and hence the associated integral \begin{align*} \int_{\gamma} \, \begin{pmatrix} 1 \\ \tau \\ \tau^2 \end{pmatrix} \frac{E_4(\tau)}{\sqrt{j(\tau)}} \, \mathrm{d} \tau \end{align*} can be obtained from $r_1$. For example, we have \begin{align*} \int_0^\infty \, \begin{pmatrix} 1 \\ \tau \\ \tau^2 \end{pmatrix} \frac{E_4(\tau)}{\sqrt{j(\tau)}} \, \mathrm{d} \tau \, = \, -\frac{1}{-24\sqrt{3}i} \, r_1(\gamma_3) \, = \, \frac{1}{24 \sqrt{3} i} \begin{pmatrix} -\lambda^+ \\ \lambda^- \\ \lambda^+ \end{pmatrix} \, . \end{align*}

Integrals of $B$

We proceed analogous to the previous part. $B_{z, 1}$ is part of a local system of rank three on $\mathcal{M}_z = \mathbb{P}^1 \setminus \phi_z^{-1}\{ 0,1,\infty \}$, where $\phi_z(t) = \frac{2^2}{3^3} \frac{z}{t \, (1-t)^2}$. We fix some $t_0 \gg 1$ and choose the following generators of $\pi_1(\mathcal{M}_z, t_0)$:

Generators of fundamental group

For the local system we can choose a vector of basis elements $B_z$ around $t_0$ such that the first component is $B_{z, 1}$ (with the branch of the square root chosen such that $1/\sqrt{1-t_0} \in i \mathbb{R}_{>0}$) and the monodromy matrices (acting by $B_z \mapsto M_{\gamma} \, B_z$) are given by $M_{\gamma_1} = -T^3, M_{\gamma_2} = -W, M_{\gamma_3} = T^{-1}WT^{-1}, M_{\gamma_4}=-T^{-2}, M_{\gamma_5}=T^{-1}WT^{-1}$ in terms of \begin{align*} T \, = \, \begin{pmatrix} 1 & 0 & 0 \\ 1 & 1 & 0 \\ 1 & 2 & 1 \end{pmatrix} \qquad \text{and} \qquad W \, = \, \begin{pmatrix} 0 & 0 & 2 \\ 0 & -1 & 0 \\ 1/2 & 0 & 0 \end{pmatrix} \, . \end{align*} Now consider the map \begin{align*} r_z : \ \pi_1(\mathcal{M}_z, t_0) \, &\rightarrow \, \mathbb{C}^3 \\ \gamma \, &\mapsto \, \int_{\gamma} B_z \, . \end{align*} The analytic continuations in $z$ give the following actions: \begin{align*} \begin{array}[h]{c|ccccc} \gamma & \gamma_1 & \gamma_2 & \gamma_3 & \gamma_4 & \gamma_5 \\ \hline M_0 \, r_z(\gamma) & T \, r_z(\gamma_1) & T \, r_z(\gamma_4\gamma_3^{-1}) & T \, r_z(\gamma_4\gamma_2^{-1}) & T \, r_z(\gamma_4) & T \, r_z(\gamma_1^{-1}\gamma_4^{-1}\gamma_5\gamma_1\gamma_4) \\ M_1 \, r_z(\gamma) & r_z(\gamma_1) & r_z(\gamma_2) & r_z(\gamma_3) & r_z(\gamma_5\gamma_4^{-1}\gamma_3) & r_z(\gamma_5) \end{array} \end{align*}

From $\Pi(z) = (1, 0, 0) \cdot r_z(\gamma_1^{-1}\gamma_4^{-1})$ and the analytic continuation we then get \begin{align*} \widetilde{\Pi}(z) \, &= \, \begin{pmatrix} 2 \\ 4 \\ -2 \end{pmatrix} \cdot r_z(\gamma_1) + \begin{pmatrix} -1 \\ 0 \\ 0 \end{pmatrix} \cdot r_z(\gamma_2) + \begin{pmatrix} 5 \\ 8 \\ 2 \end{pmatrix} \cdot r_z(\gamma_4) + \begin{pmatrix} -1 \\ 0 \\ 0 \end{pmatrix} \cdot r_z(\gamma_5) \\ &= \, \begin{pmatrix} 2 \\ 4 \\ -2 \end{pmatrix} \cdot r_z(\gamma_1) + \begin{pmatrix} -1 \\ 0 \\ 0 \end{pmatrix} \cdot r_z(\gamma_2) + \begin{pmatrix} 4 \\ 8 \\ 2 \end{pmatrix} \cdot r_z(\gamma_5) \, . \end{align*} Here, we used that $(5,8,2) \cdot r_z(\gamma_4) = (5,8,2) \cdot r_z(\gamma_5)$ since the integrand is holomorphic around $t_{\pm}$ and hence the countour $\gamma_4$ can be pushed outside of the region that gets pinched for $z \rightarrow 1$.

As a last step, we consider the modularity of $B$. For this, consider the modular function \begin{align*} t_2(\tau) \, = \, \frac{(\eta(2\tau)/\eta(\tau))^{24}}{(1+64 \, (\eta(2\tau)/\eta(\tau))^{24})^2} \end{align*} and the modular function $t_{72}$ which solves $\phi_1(t_{72}(\tau)) = 256 \, t_2(6\tau)$ and has the Fourier expansion $t_{72}(\tau) = \frac{1}{12}q^{-2}+O(q^0)$. Pulling back by $t_{72}$ we get \begin{align*} (t_{72}^*B)(\tau) \, = \, -4\sqrt{3}i \, \begin{pmatrix} 1 \\ 6\tau \\ (6\tau)^2 \end{pmatrix} (f(\tau)-9 \, f(3\tau))\, \mathrm{d}\tau \end{align*} around $\tau = \infty$. For $z = 1$ we thus get \begin{align*} \widetilde{\Pi}(1) &= \lim_{t_0 \rightarrow \infty} \left( \begin{pmatrix} -1 \\ 0 \\ 0 \end{pmatrix} \cdot r_z(\gamma_2) + \begin{pmatrix} 4 \\ 8 \\ 2 \end{pmatrix} \cdot r_z(\gamma_5) \right) \\ &= -4\sqrt{3}i \left( \begin{pmatrix} -1 \\ 0 \\ 0 \end{pmatrix} \int_{\infty}^0 \begin{pmatrix} 1 \\ 6\tau \\ (6\tau)^2 \end{pmatrix} (f(\tau)-9 \, f(3\tau)) \mathrm{d}\tau + \begin{pmatrix} 4 \\ 8 \\ 2 \end{pmatrix} \int_{\infty}^{-1/6} \begin{pmatrix} 1 \\ 6\tau \\ (6\tau)^2 \end{pmatrix} (f(\tau)-9 \, f(3\tau)) \mathrm{d}\tau \right) \end{align*} and it is a straightforward exercise in period polynomials to show that the latter evaluates to $-24\sqrt{3}i \, (4\, \omega^- + 2 \, \omega^+)$.

Final steps

In our study of the integrals of $A$ we have constructed $r^\pm(\gamma) \in \mathbb{Z}^3$ such that \begin{align*} \int_{\gamma} \, \begin{pmatrix} 1 \\ \tau \\ \tau^2 \end{pmatrix} \frac{E_4(\tau)}{\sqrt{j(\tau)}} \, \mathrm{d} \tau \, = \, \frac{1}{-24\sqrt{3}i} \left(\lambda^- \, r^+(\gamma) + \lambda^+ \, r^-(\gamma) \right) \, , \end{align*} where $\lambda^+ \in \mathbb{R}$ and $\lambda^- \in i\mathbb{R}$ are given by $\widetilde{\Pi}(1) = 4\, \lambda^+ + 2 \, \lambda^-$. In our study of the integrals of $B$ we have further shown that \begin{align*} \widetilde{\Pi}(1) = -24\sqrt{3}i \, (4\, \omega^- + 2 \, \omega^+) \end{align*} with the periods $\omega^\pm$ of $f$. Hence $\lambda^\pm = -24\sqrt{3}i \, \omega^\mp$, which finishes the proof. Note that, by evaluating other analytic continuations and derivatives of $\Pi$ at $z=1$, one can get further identities e.g. featuring quasiperiods of $f$.

user1363745
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