code: def E_nl(n, Z=1):
      """
      Returns the energy of the state (n, l) in Hartree atomic units.
  
      The energy does not depend on "l".
  
      Parameters
      ==========
  
      n : integer
          Principal Quantum Number which is
          an integer with possible values as 1, 2, 3, 4,...
      Z :
          Atomic number (1 for Hydrogen, 2 for Helium, ...)
  
      Examples
      ========
  
      >>> from sympy.physics.hydrogen import E_nl
      >>> from sympy.abc import n, Z
      >>> E_nl(n, Z)
      -Z**2/(2*n**2)
      >>> E_nl(1)
      -1/2
      >>> E_nl(2)
      -1/8
      >>> E_nl(3)
      -1/18
      >>> E_nl(3, 47)
      -2209/18
  
      """
      n, Z = S(n), S(Z)
      if n.is_integer and (n < 1):
          raise ValueError("'n' must be positive integer")
      return -(Z**2) / (2 * n**2)
  
file_path: sympy/sympy/physics/hydrogen.py
imports: from sympy.core.numbers import Float
  from sympy.core.singleton import S
  from sympy.functions.combinatorial.factorials import factorial
  from sympy.functions.elementary.exponential import exp
  from sympy.functions.elementary.miscellaneous import sqrt
  from sympy.functions.special.polynomials import assoc_laguerre
  from sympy.functions.special.spherical_harmonics import Ynm
label_desc: Use image node_id 3 for calling a global function with example usage: E_nl(n, Z) and returns: unknown