In number theory, specifically the study of modular forms, a Maass–Shimura operator is an operator which maps modular forms to almost holomorphic modular forms.
Definition The Maass–Shimura operator on (almost holomorphic) modular forms of weight k {\displaystyle k} is defined by
δ k f ( z ) := 1 2 π i ( k 2 i y + ∂ ∂ z ) f ( z ) {\displaystyle \delta _{k}f(z):={\frac {1}{2\pi i}}\left({\frac {k}{2iy}}+{\frac {\partial }{\partial z}}\right)f(z)}
where y {\displaystyle y} is the imaginary part of z {\displaystyle z} . One may similarly define Maass–Shimura operators of higher orders, where
δ k ( n ) := δ k + 2 n − 2 δ k + 2 n − 4 ⋯ δ k + 2 δ k = 1 ( 2 π i ) n ( k + 2 n − 2 2 i y + ∂ ∂ z ) ( k + 2 n − 4 2 i y + ∂ ∂ z ) ⋯ ( k + 2 2 i y + ∂ ∂ z ) ( k 2 i y + ∂ ∂ z ) , {\displaystyle {\begin{aligned}\delta _{k}^{(n)}&:=\delta _{k+2n-2}\delta _{k+2n-4}\cdots \delta _{k+2}\delta _{k}\\&={\frac {1}{(2\pi i)^{n}}}\left({\frac {k+2n-2}{2iy}}+{\frac {\partial }{\partial z}}\right)\left({\frac {k+2n-4}{2iy}}+{\frac {\partial }{\partial z}}\right)\cdots \left({\frac {k+2}{2iy}}+{\frac {\partial }{\partial z}}\right)\left({\frac {k}{2iy}}+{\frac {\partial }{\partial z}}\right),\end{aligned}}}
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