"Calculator forensics" is a term (credit to Mike Sebastian) which seeks to answer the questions of who designed a particular calculator's chipset initially, what features of a specific calculator have been borrowed from earlier designs, and how has calculator technology spread among the manufacturers.
The Forensics Evaluation Algorithm
Observations regarding the embedded algorithms are useful in identifying related calculators. The evaluation algorithm chosen is (with the calculator in degrees mode):
arcsin (arccos (arctan (tan (cos (sin (9) ) ) ) ) )
The keystroke sequence (ignoring 2nd, INV, or ARC keys) is typically 9, sin, cos, tan, arctan, arccos, arcsin.
For the HP calculators with RPN notation, the keystroke is typically the same.
This algorithm usually produces different results on different calculator models.