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In mathematics, a complex number is an element of a number system that extends the real numbers with a specific element denoted i, called the imaginary unit and satisfying the equation . Since no real number satisfies the above equation, i was called an imaginary number by René Descartes. Every complex number can be expressed in the form , where a and b are real numbers, a is called the real part, and b is called the imaginary part. The set of complex numbers is denoted by either of the symbols or C. Despite the historical nomenclature, "imaginary" numbers are not fictitious: they are no less real mathematically than real numbers, and they are essential to the scientific description of the physical world.