#equation  a mathematical statement that two expressions are equal
  supertype:  #mathematical_statement  a statement of a mathematical relation
  subtype:  #regression_equation__regression_of_y_on_x  the equation representing the relation between selected values of one variable (x) and observed values of the other (y); it permits the prediction of the most probable values of y
  subtype:  #linear_equation  a polynomial equation of the first degree
  subtype:  #quadratic_equation__quadraticequation__quadratic  an equation in which the highest power of an unknown quantity is a square
  subtype:  #biquadratic_equation__biquadraticequation__biquadratic  and equation of the fourth degree
  subtype:  #differential_equation  an equation containing differentials of a function
     subtype:  #Maxwell's_equations  four differential equations that summarize classical properties of electromagnetic fields
     subtype:  #partial_differential_equation  a differential equation involving a functions of more than one variable
     subtype:  #Schrodinger_equation__Schrodinger_wave_equation  the fundamental equation of wave mechanics
     subtype:  #wave_equation  a differential equation that describes the passage of harmonic waves through a medium
  subtype:  #exponential_equation__exponentialequation  an equation involving exponential functions of a variable

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