∀ (γ : Matrix.SpecialLinearGroup (Fin 2) ℤ) (z : UpperHalfPlane),
  EisensteinSeries.D2 γ z =
    2 * ↑Real.pi * Complex.I * ↑(↑γ 1 0) /
      UpperHalfPlane.denom (Matrix.SpecialLinearGroup.toGL ((Matrix.SpecialLinearGroup.map (Int.castRingHom ℝ)) γ)) ↑z