I installed HP 15C calculator emulator on my iPod Touch. But I do not have a manual to learn how to use it. So I have to figure out how it works by trying. When I tapped f key and I/TAN/TAN-1 key, a letter c was displayed on the display. What mode is the calculator in? And how to exit it? I searched the internet and found out that this mode is complex mode. In this mode, it can manipulate complex numbers. But I still want to know how to exit this mode.