Happy anniversary, HP-25!

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I never owned a HP-25 but for me the importance of this model is Peter Henrici's book Computational Analysis with the HP-25 Pocket Calculator which taught me much about numerical analysis and programming.

I ported most of Henrici's programs to the HP-21S I had. The 21S was a model of similar capability to the 25 with 10 registers (vs 8) and 99 program steps (vs 49). Though it was algebraic logic and so less efficient than RPN as too many of those 99 steps tended to get wasted on parenthesis.

Later I implemented Henrici's programs on a 42S which had a enough memory I could extend them and keep many of them on hand as a numerical library. That wonderful device helped get me through an MSc.

Today I have go25c on my Android phone but that is more for nostalgia, my main calculator is Thomas Okken's splendid Free42.

Happy Anniversary to the HP-25 indeed!