The $j$-ivariant has the following Fourier expansion
$$j(\tau)=\frac 1q +\sum\_{n=0}^{\infty}a\_nq^n=\frac{1}{q}+744+196884q+21493760q^2+\cdots.$$
Here is $q=e^{2\pi i \tau}$. 


Is there some simple **effective** bound on the coefficients $a\_n$?


**Backround.**


This question comes from [On the “gap” in a theorem of Heegner](https://www.sciencedirect.com/science/article/pii/0022314X69900237). Let $D$ be a negative discriminant such that $h(D)=1$. We want to show that $J=j(\sqrt{D})$ generates a cubic extension of $\mathbf Q$. Since we have at our disposal a monic cubic polynomial with rational coefficients, the modular equation $\Phi\_2(X,j)$, whose root is $J$, and the other two roots are non-real, it is sufficient to show that $J$ is not an integer.


In this case $j=j\left(\frac{-1+\sqrt D}{2} \right)$ is also an integer. Set


$$t=e^{2\pi i(-1+\sqrt D)/2}.$$


Then 


$$J=\frac{1}{t^2}+744+196884t^2+O(t^4),$$


and 


$$j^2-1488j+160512-J=42987520t+O(t^2).$$.


On the left there is an integer. However the right side tends to zero as $|D|$ gets large. Stark asserts that $|D|>60$ is enough for the RHS to be less than 1. **Why is it enough?**



Effective bound on the expansion of the $j$-invariant
Once you know that the coefficients are all positive (see postscript),
it's easy to get an effective upper bound that grows as $\exp(4\pi \sqrt{n})$,
which is within a factor $O(\sqrt n)$ of the correct order of growth. 
Start from the inequality
$$
a\_n = q^{-n} (a\_n q^n) < q^{-n} \sum\_{k=-1}^\infty a\_k q^k = q^{-n} j(\tau)
$$
for any purely imaginary $\tau = it$ (because $q = e^{-2\pi t} > 0$
so each term $a\_n q^n$ is positive). If $t \geq 1$ then
$$
j(it) = e^{2\pi t} + \sum\_{n=0}^\infty a\_n e^{-2\pi n t}
\leq e^{2\pi t} + \sum\_{n=0}^\infty a\_n e^{-2\pi n}
= e^{2\pi t} + j(i) - e^{2\pi} < e^{2\pi t} + 1728.
$$
Since $j(i/t) = j(it)$ it follows that also
$$
j(it) < e^{2\pi/t} + 1728
$$
for $t \leq 1$. Thus our inequality on $a\_n$ yields
$$
a\_n < q^{-n} j(\tau) < e^{2\pi n t} (e^{2\pi/t} + 1728).
$$
The main term $\exp(2\pi (nt+1/t))$ is minimized at $t = 1/\sqrt{n}$
where it equals $\exp(4\pi \sqrt{n})$. Choosing this value of $t$ yields
$$
a\_n < e^{4\pi \sqrt n} + 1728 e^{2\pi \sqrt n}
$$
which is an effective bound of the desired kind.


Postscript: one easy proof of $a\_n>0$ starts from the formula
$j = E\_4^3 / \Delta$: the coefficients of $E\_4$ are all positive,
so the same is true for $E\_4^3$; and 
$1 / \Delta = q^{-1} \prod\_{m=1}^\infty ((1-q^m)^{-1})^{24}$
where each factor has nonnegative coefficients because
$(1-q^m)^{-1} = \sum\_{k=0}^\infty q^{km}$. So the product
$E\_4^3 \cdot 1/\Delta$ also has positive coefficients.


