#operator  (mathematics) a symbol that represents a function from functions to functions; "the integral operator"
  supertype:  #mathematical_function__function  a mathematical relation such that each element of one set is associated with at least one element of another set
  subtype:  #linear_operator  an operator that obeys the distributive law: A(f+g) = Af + Ag (where f and g are function)
  subtype:  #identity_operator__identity__identity_element  an operator that leaves unchanged the element on which it operates; "the identity under numerical multiplication is 1"

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