Hi, Herbert;
this small C means Complex Mode active. If you press [f][I] or [f][Re<>Im], it is activated and so the C annunciator. Maybe it happened and you did not notice.
As Rn Ross mentioned, you can deactivate it with the sequence:
[g] [CF] 8 ([CF] is the blue inscription for the [5] key)
If you leave it there, you'll get automatic complex results for operations where real results do not apply, like:
_
2 [CHS] [Vx] (square root)
You'll not get [Error 0] message if your HP15C is in complex mode (small C on), instead a zero (real part) in the display. To see the imaginary part, you press (and hold for as long as you want to):
[f] [(i)]
Hope this adds a bit more of information.
Best regards.
Luiz (Brazil)
Edited: 29 Dec 2003, 9:37 p.m.