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 is the imaginary unit of any complex number, discovered by the Italian mathematician Girolamo Cardano.

A purely imaginary is a number in the form , where  is a real number and  is the square root of  for .

The complex numbers in general, and the purely imaginary numbers, in particular, are essential to describe physical phenomena and has concrete applications in electromagnetism, signal processing, control theory, quantum mechanics, cryptography, cartography, etc..

 is the result of the following equations:


, for .

Square roots of negative numbers can be written in the form: .

Another form of representing the number :

Powers of  repeat a particular pattern :