A **complex number** is a number that can be put in the form *a* + *bi*, where *a* and *b* are real numbers and *i* is called the imaginary unit, where *i*2 = −1. In this expression, *a* is called the real part and *b* the imaginary part of the complex number. Complex numbers extend the idea of the one-dimensional number line to the two-dimensional complex plane by using the horizontal axis for the real part and the vertical axis for the imaginary part. The complex number *a* + *bi* can be identified with the point (*a*, *b*). A complex number whose real part is zero is said to be purely imaginary, whereas a complex number whose imaginary part is zero is a real number. In this way the complex numbers contain the ordinary real numbers while extending them in order to solve problems that cannot be solved with only real numbers.

Complex numbers are used in many scientific fields, including engineering, electromagnetism, quantum physics, and applied mathematics, such as chaos theory. Italian mathematician Gerolamo Cardano is the first known to have introduced complex numbers. He called them "fictitious", during his attempts to find solutions to cubic equations in the 16th century.

Read more about Complex Number: Overview, Complex Analysis, History, Generalizations and Related Notions

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**Complex Number**- Generalizations and Related Notions

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