1.2. INTEGER WEIGHT MODULAR FORMS 3
1.2. Integer weight modular forms
The group
GL2
+
(R) = γ =
a b
c d
: a, b, c, d R and ad bc 0
acts on functions f(z) : H C. In particular, suppose that γ =
a b
c d

GL2
+
(R). If f(z) is a meromorphic function on H and k is an integer, then define
the “slash” operator |k by
(1.3) (f|kγ) (z) := (det
γ)k/2(cz
+
d)−kf(γz),
where
γz :=
az + b
cz + d
.
Definition 1.8. Suppose that f(z) is a meromorphic function on H, that
k Z, and that Γ is a congruence subgroup of level N. Then f(z) is called a
meromorphic modular form with integer weight k on Γ if the following hold:
(1) We have
f
az + b
cz + d
= (cz +
d)kf(z)
for all z H and all
a b
c d
Γ.
(2) If γ0 SL2(Z), then (f|kγ0) (z) has a Fourier expansion of the form
(f|kγ0) (z) =
n≥nγ0
aγ0 (n)qN
n
,
where qN := e2πiz/N and aγ0 (nγ0 ) = 0.
If k = 0, then f(z) is known as a modular function on Γ.
Remark 1.9. Note that condition (2) in Definition 1.8 can refer to a Fourier
series involving fractional powers of qN . More specifically, for “irregular cusps”
these series can be thought of as expansions in
qN/2. 1
The reader should consult
page 29 of [Shi1] for a detailed discussion.
Remark 1.10. Condition (2) of Definition 1.8 means that f(z) is meromorphic
at the cusps of Γ. If nγ0 0 (resp. nγ0 0) for each γ0 SL2(Z), then we say
that f(z) is holomorphic (resp. vanishes) at the cusps of Γ.
Remark 1.11. Since
−1 0
0 −1
Γ0(N), there are no nonzero meromorphic
modular forms of odd weight k on Γ0(N).
Definition 1.12. Suppose that f(z) is an integer weight meromorphic modular
form on a congruence subgroup Γ. We say that f(z) is a holomorphic modular (resp.
cusp) form if f(z) is holomorphic on H and is holomorphic (resp. vanishes) at the
cusps of Γ. We say that f(z) is a weakly holomorphic modular form if its poles (if
there are any) are supported at the cusps of Γ.
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