Minimal Key Calculator

[Gerald H]

In another thread the question of unnecessary functions & redundant keys is raised:

http://www.hpmuseum.org/forum/thread-3345.html

What could be the least number of keys needed to reproduce all the functions of, say, the HP 35S keyboard?

If you were happy with a purely binary calculator, only two number keys would be required - but probably you would want to keep decimal input, so let's say you stay decimal.