Trig algorithms on HP RPN?

[Thomas Klemm]

(12-01-2024 07:27 PM)Albert Chan Wrote:  Just a guess, if angles are in radian, we can just use tan code to get atan.

This is already the case.
From A Unified Algorithm for Elementary Functions:

J. S. WALTHER Wrote:This paper describes a single unified algorithm for the calculation of elementary functions including multiplication, division, sin, cos, tan, arctan, sinh, cosh, tanh, arctanh, ln, exp and square-root.