= Solution
Put $\tau=x+iy$. The modular transformation law and
$$
\operatorname{Im}(\gamma\tau)=\frac{y}{|c\tau+d|^2}
$$
show that the <invariant norm of a modular form>
$$
y^{k/2}|f(\tau)|
$$
is invariant under $\Gamma(1)$. On the region from part (a), it is bounded: it is continuous on every truncated region, while the <cusp form> condition makes it tend to zero as $y\to\infty$. Thus
$$
|f(x+iy)|\leq C_0y^{-k/2}
$$
for all $x\in\mathbb R$ and $y>0$.
The <Fourier coefficient> formula on one period gives
$$
a_n=e^{2\pi ny}\int_0^1f(x+iy)e^{-2\pi inx}\,dx,
$$
and hence
$$
|a_n|\leq C_0e^{2\pi ny}y^{-k/2}.
$$
Choosing $y=1/n$ yields
$$
|a_n|\leq C_0e^{2\pi}n^{k/2}.
$$
This proves the <Fourier coefficient bound for a cusp form>.
Back to article page
