For the historical record and in order to preserve my earliest and/or unreleased efforts in the wonderful world of HP programmable calculators and pocket computers, here you'll find a selection of HP programs I wrote for various HP models ranging in capabilities from the HP-25 to the HP-71B.
The HP-25 programs are for the most part (not all) quite simple affairs, written just for practice and never intended for release, as back in 1975 I was learning programming techniques (the HP-25 was my very first programmable calc) and the HP-25 was slow and had very little RAM (49 steps, 8 registers) as well as a very limited programming paradigm (no labels, no subroutines, no flags), so don't expect anything too sophisticated (though there are a few surprises). That said, I think they're still quite enjoyable and fun to check and even convert to run on other devices.
On the other hand, most subsequent HP models (such as the HP-11C, HP-67, HP-34C, HP-41C, etc., let alone the HP-71B !) had much more RAM for both programs and data and also advanced programming capabilities, so the programs I wrote for them take full advantage of that and are significantly more complex and capable, no longer "practice" ones. Some of them were published in the past in physical media (paper magazines, solution books, CD/DVD/USB pendrives), which would-be readers (even the authors themselves) had to pay for, but now for the first time they'll be available to download for free, as will existing but still unpublished programs or even new programs I might write now and in the foreseeable future. Stay tuned !
3-page paper featuring a 37-step RPN program for the HP-25 to numerically solve 2nd-order differential equations of the form y”=f(x,y) subject to initial conditions, using the 5th-order predictor-corrector Numerov’s method. One worked example included.
2-page paper featuring a 49-step RPN program for the HP-25 to accurately evaluate the Gamma function in the interval [1, 2], as well as approximate factorials for real-valued x. Three worked examples included.
2-page paper featuring a 70-step RPN program for the HP-34C to generate either a complete amortization schedule showing each period of a fully amortized loan or a partial schedule between two given periods. One worked example is included.
3-page paper featuring an 84-step RPN program for the HP-34C to quickly and accurately find the sum of infinite alternating series, even divergent ones (Euler sum). Three worked examples are included.
3-page paper featuring a 67-step RPN program for the HP-34C to compute the numeric value of a definite double integral of a user-specified f(x,y) between given limits. Four worked examples are included.
4-page paper featuring a 42-step RPN program for the HP-41C to find real roots of an arbitrary user-supplied equation f(x)=0 using Newton’s method and a user-given initial guess. Interactive and non-interactive versions provided. Five worked examples included.
3-page paper featuring a 28-step RPN program for the HP-41C to find extrema (maxima and/or minima) of an arbitrary user-supplied function y=f(x) by calling program RF (Root Finder) internally as part of the computation. Two worked examples are included.
3-page paper featuring a 55-step RPN program for the HP-41C to evaluate the definite integral between given limits of an arbitrary user-supplied function f(x) using the fast 3-point Gauss-Legendre quadrature formula applied over a number of subintervals. Three worked examples included.
3-page paper featuring a 142-step fun game RPN program for the HP-41C to challenge your memory by testing your ability to remember what you’ve just seen and offering afterwards an accurate comment on your performance.
2-page paper featuring an 85-step fun RPN program for the HP-41C to test your “ESP” (Extra-Sensorial Powers) by conducting a series of 10 tests, after which it reports the % of success and an evaluation of your alleged extrasensorial abilities (if any). One sample run included.
3-page paper featuring a 5-line BASIC subprogram for the HP-71B to evaluate determinants and permanents for real or complex NxN matrices. Unlike other methods where floating-point divisions are involved, if the elements are moderately-sized integers the integer results will be exact, even for singular or very ill-conditioned matrices. Two worked examples are included.
These articles, programs, pictures, their descriptions and other materials created by me are (c) Valentin Albillo, and can be used freely for non-profit purposes as long as (1) the contents aren't modified in any way and (2) the copyright is acknowledged.
In plain words, you can download them and use them for non-profit purposes but do not include them in any media and/or site for which you're asking money, do not tamper with their contents and do not say or imply that you created them or that you don't know who created them, you must always give due credit to the copyright holder (that's me).