HP42s first major program (Double Integral) Best way to approach?

[John Keith]

(05-27-2020 07:16 AM)J-F Garnier Wrote:  The HP-71B Math ROM was the first to allow recursive solve or integrate functions, up to 5 levels. It was a major design challenge, as related in the HP Journal article.

Thanks for the reference, it is very informative. I would highly recommend users of later HP calculators, from the HP-28 through the Prime, read this as well. It explains the operation of these commands and PROOT (polynomial roots) as well as the underlying algorithms.