This week I have been using the HP15c in my normal work. for that I have entered several financial programs including the Time Value of Money program. It is a long program but it makes use of many of the programming techniques in the HP15c.
The HP15c always surprises me regarding how cleverly distributed the keyboard is, and how well thought out is the function set. Most of the functions can be applied to complex arguments; also, compared with other HP calculators (including the advantage pack for the HP41c), the complex mode doubles the RPN stack and so programs can be applied to real or complex numbers, depending on which mode the calculator is; it allows to redefine the upper 5 keys; and you can combine complex numbers with the integrate and solver features.
A surprising capability on a calculator this size is the extended use of matrixes - you can invert matrixes and can also use them to solve linear systems - which is something you can do in an exam whithout anyone noticing that you're using your calculator. Also I liked how elegant is to solve an equation system: after entering the 3 matrix dimensions (coefficients, independent terms and results), you enter the values for each term in the matrix, and then you recall the independent term vector, you recall the coefficient matrix, and then you just press the divide key! then you have the result in the results matrix. So powerful, and yet so simple and elegant.
The calculator came initially with two manuals: one was the user manual, roughly equivalent to that of the other calculators (but with many more functions, of course); however, this was a machine devoted to scientists and engineers, and it should explain them with details how it works and what are the limits of operation and accuracy in the different functions and features. So, the machine came with an "Advanced Functions Handbook", pictured besides.
The manual dealt with the following topics (reading from the index):
These subjects are dealt with detail and combining precision and clarity - a must read for any computer science student!
The previous anniversary version did not have this manual, only the User Manual. Let's hope that, if this calculator is ever resurrected, it comes with this manual too!