Complex conjugation

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In mathematics, complex conjugation is an operation on complex numbers which reverses the sign of the imaginary part, that is, it sends $z = x + iy$ to the complex conjugate $\bar z = x-iy$ .

In the geometrical interpretation in terms of the Argand diagram, complex conjugation is represented by reflection in the x-axis. The complex numbers left fixed by conjugation are precisely the real numbers.

Conjugation respects the algebraic operations of the complex numbers: $\overline{z+w} = \bar z + \bar w$ and $\overline{zw} = \bar z \bar w$ . Hence conjugation represents an automorphism of the field of complex numbers over the field of real numbers, and is the only non-trivial automorphism. One can say it is impossible to tell which is i and which is -i.