HP 15c - Manual (Page 121)

Section 11: Calculating With Complex Numbers 121
Complex mode is activated
1) automatically, when executing ´ V or ´ }; or
2) by setting flag 8, the Complex mode flag (|F 8).
When the calculator is in Complex mode, the C annunciator in the display
is lit. This tells you that flag 8 is set and the complex stack exists. In or out
of Complex mode, the number appearing in the display is the number in the
real X-register.
Note: In Complex mode (signified by the C annunciator), the HP-
15C performs all trigonometric functions using radians. The
trigonometric mode annunciator in the display (RAD, GRAD, or
blank for Degrees) applies to two functions only: ;
and :
(as explained later in this section).
Deactivating Complex Mode
Since Complex mode requires the allocation of five registers from memory,
you will have more memory available for programming and other advanced
functions if you deactivate Complex mode when you are working solely
with real numbers.
To deactivate Complex mode, clear flag 8 (keystroke sequence: | "
8). The C annunciator will disappear.
Complex mode is also deactivated when Continuous Memory is reset (as
described on page 63). In any case, deactivating Complex mode dissolves
the imaginary stack, and all imaginary numbers there are lost.
Complex Numbers and the Stack
Entering Complex Numbers
To enter a number with real and imaginary parts;
1. Key the real part of the number into the display.
2. Press v
3. Key the imaginary part of the number into the display.
4. Press ´ V. (If not already in Complex mode, this creates the
imaginary stack and displays the C annunciator.)