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Let $j=12^3 E_4^3/(E_4^3-E_6^2)$ be the modular $j$-invariant $$j(\tau)=q^{-1} + 744 + 196884q +...,$$ with $q=e^{2\pi i \tau}$ and $E_k$ the weight $k$ Eisenstein series. At the elliptic points $\tau=i$ and $\tau=\zeta_3=e^{2\pi i/3}$ of $\text{PSL}(2,\mathbb Z)$, it has a special behaviour: $\tau=\zeta_3$ is a third order zero of $j$, while $\tau=i$ is a second order zero of $j-12^3$. Thus near those elliptic points the behaviour of $j$ is determined through the first nonzero coefficients in the Taylor series, which are $j'''(\zeta_3)$ and $j''(i)$.

Using some experimental numerics, I found that $$j'''(\zeta_3)=-\frac{162i \Gamma(\frac13)^{18}}{\pi^9}, \\ j''(i)=-\frac{162\Gamma(\frac14)^8}{\pi^4}.$$

I have not spotted these values in the literature, but they must have been found before. I would be very glad about any hints on the literature of such computations, as it would be great to confirm these numbers and learn about how they can be computed in similar examples (of say other modular forms). Many thanks!

El Rafu
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    You want to find $E_4',E_6'$ at $i,e^{2i\pi/3}$. In the book "the 1-2-3 of modular forms" (on libgen) there is a chapter on derivatives of modular forms, the main theorem being that $g=f'-\frac{k}{12} E_2 f$ is a weight $k+2$ modular form. So it must be that $g = c E_{k+2}$ for $f=E_k$. So it is now about finding $E_6(e^{2i\pi/3})$ and $E_8(i)= E_4(i)^2$. – reuns Dec 29 '22 at 00:16
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    This is a well known exercice, linking $\int_0^1 dz$ on the relevant complex torus to $\int_\gamma \frac{dx}{\sqrt{x^3+x}}$ and $\int_\gamma \frac{dx}{\sqrt{x^3-1}}$ for some closed loop on the relevant elliptic curve, which can be evaluated in term of the Beta function whence in term of your Gamma constants. – reuns Dec 29 '22 at 00:18
  • Thank you! I understand that everything boils down to calculating $E_6(\zeta_3)$ and $E_4(i)$. Can you give me a hint on how those are related to elliptic integrals? I'm not quite familiar with those. – El Rafu Dec 29 '22 at 10:41

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I will use the notation by Ramanujan with $E_2, E_4,E_6$ being denoted by $P, Q, R$. For clarity let us write \begin{align} P(q) & =1-24\sum_{n=1}^{\infty}\frac {nq^n} {1-q^n}\tag{1a}\\ Q(q) &=1+240\sum_{n=1}^{\infty}\frac{n^3q^n}{1-q^n}\tag{1b}\\ R(q) &=1-504\sum_{n=1}^{\infty}\frac{n^5q^n}{1-q^n}\tag{1c}\end{align} where $q=\exp(2\pi i\tau) $ and $\tau$ has positive imaginary part. Let us also list Ramanujan's identities dealing with derivatives of these functions \begin{align} q\frac{dP} {dq} &=\frac{P^2-Q}{12}\tag{2a}\\ q\frac{dQ}{dq}&=\frac{PQ-R}{3}\tag{2b}\\ q\frac{dR}{dq}&=\frac{PR-Q^2}{2}\tag{2c} \end{align} Further let us observe that $$\frac{dq} {d\tau} =2\pi iq$$ and hence $$\frac{d} {d\tau} =2\pi i q\frac{d} {dq} \tag{3}$$ We have by definition $$j(\tau) =\frac{12^3Q^3}{Q^3-R^2}\tag{4}$$ and one can prove using the equations $(2a),(2b),(2c)$ that $$q\frac{d} {dq} (Q^3-R^2)=P(Q^3-R^2)\tag{4}$$ Hence we have $$j'(\tau) =12^3\cdot 2\pi i q\frac{d} {dq} \frac{Q^3}{Q^3-R^2}=12^3(2\pi i) \cdot \dfrac{(Q^3-R^2)3Q^2q\dfrac{dQ}{dq}-Q^3q\dfrac{d}{dq}(Q^3-R^2)} {(Q^3-R^2)^2}=-12^3(2\pi i) \cdot\frac{Q^2R}{Q^3-R^2}$$ Differentiating again we get $$j''(\tau) =12^2(8\pi^2)\cdot\frac{PQ^2R-4QR^2-3Q^4}{Q^3-R^2}\tag{5}$$ One more round of differentiation gives us $$j'''(\tau) =12(64\pi^3 i)\cdot\frac{4R^3+\text{ terms containing }Q }{Q^3-R^2}\tag{6}$$ Next we use the link between $Q, R$ and elliptic integrals to evaluate them in closed form. Let $0<k<1$ be the elliptic modulus and $k'=\sqrt{1-k^2}$ be complementary modulus and let $K, K'$ be elliptic integrals defined by $$K=K(k) =\int_0^{\pi/2}\frac{dx}{\sqrt{1-k^2\sin^2x}},K'=K(k')\tag{7}$$ Let $q'=\exp(-\pi K'/K) $ be the corresponding nome and then we have the standard formulas (proved here using the identities $(2a),(2b),(2c)$ mentioned above) \begin{align} Q(q'^2) &=\left(\frac{2K}{\pi}\right) ^4(1-k^2k'^2)\tag{8a}\\ R(q'^2) &=\left(\frac{2K}{\pi}\right)^6(1-2k^2)\left(1+\frac{k^2k'^2}{2}\right)\tag{8b}\\ Q(-q')&=\left(\frac{2K}{\pi}\right)^4(1-16k^2k'^2)\tag{8c}\\ R(-q')&=\left(\frac{2K}{\pi}\right)^6(1-2k^2)(1+32k^2k'^2)\tag{8d} \end{align} Further let us note that if $N$ is a positive rational number then there exists a unique positive value of modulus $k$ such that $K'/K=\sqrt{N} $ and this value $k$ is an algebraic number. Such $k$ are famously known as singular moduli. For small values of positive integer $N$ the values of $k=k_N$ as well as that of $K=K_N$ are well known. For our purposes we need to deal with $N=1,3$ and we have $$k_1=\frac{1}{\sqrt{2}},K_1=\frac {\Gamma ^2(1/4)}{4\sqrt{\pi}},k_3=\frac{\sqrt{3}-1}{2\sqrt{2}},K_3=\frac{3^{1/4}\Gamma ^{3}(1/3)}{2^{7/3}\pi} \tag{9}$$ (see details in this thread).

We can now evaluate $j''(i), j'''(\zeta_3)$ using the above given formulas. Let $\tau=i$ so that $q=e^{-2\pi}$ and then we can use the formulas $(8a),(8b),(9)$ with $q' =e^{-\pi}, k=k_1,K=K_1$ and note that $R=0$ and using $(5)$ we get desired value of $j''(i) $.

Next we use $\tau=\zeta_3$ so that $q=-e^{-\pi\sqrt{3}}$ and we use $(8c),(8d),(9)$ with $q' =e^{-\pi\sqrt{3}},k=k_3,K=K_3$ and note that $Q=0$ and using $(6)$ we get the desired value of $j''' (\zeta_3)$.