**$Complex ON/OFF i/j**

EES provides an option in the Complex tab of the Preferences dialog to enable or disable complex algebra. This directive provides a means of setting the option for complex numbers in the Equations Window. The directive also optionally allows whether i or j is to be used to represent the square root of -1. To allow complex algebra with j as the square root of -1, enter

$Complex On j

To disable it, enter

$Complex Off

The complex mode can also be turned on or off by clicking in the Complex pane in the status bar at the bottom of the Equations window.

The $Complex On directive can be used in Subprograms. It is possible to have the main program operate without complex numbers and call a Subprogram that does the complex calculations. Note that he real and imaginary parts of a complex number must be passed to the Subprogram separately. The real part is identified with _r at the end of the variable name. The imaginary part has a _i at the end of its variable name.

Modules and functions cannot be used in a program that uses complex calculations.