Scientific Calculator Keyboard Challenge

[sa-penguin]

How about... six buttons?
Up, Down, Left, Right, Select [Enter], and Escape [backspace].
After that, it's just a case of having a massive menu / submenu / function - selection system, written to the LCD.

Sure, you could expand out with ten digits and a decimal place, for a total of 17 keys. You could have an on/off button, instead of a switch on the side [or removing the battery to turn it off]. I don't recall anyone saying this layout had to PRACTICAL.

Heck, if the menus "wrap around" you could even eliminate the Up & Left keys, leaving you with a four-key calculator. If pushing Right and Down at the same time triggers the escape, you could even get it down to three.