 If formula_3 is a polynomial in two variables, the corresponding curve is called an "algebraic curve", and specific methods are available for studying it.Plane curves can be represented in Cartesian coordinates ("x", "y" coordinates) by any of three methods, one of which is the implicit equation given above.The graph of a function is usually described by an equation formula_4 in which the functional form is explicitly stated; this is called an "explicit" representation.