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# Imaginary unit

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The **imaginary unit** is one of the four units of the domain of complex numbers. (The others are 1, –1 and − *i*). In electrical engineering applications, the symbol *j* is generally used instead of *i*, which is then generally used as an iterator.^{[1]} The imaginary unit is ** I** in Wolfram Mathematica's InputForm and a blackboard bold lowercase I in StandardForm.

The square root of the imaginary unit is (see A010503).

- ↑ Paul J. Nahin,
*An Imaginary Tale: The Story of*. Princeton, New Jersey: Princeton University Press (1988): 125