By Erik Ehrling (Sweden). Any comments or suggestions are very welcome!

This program is supplied without representation or warranty of any kind. Erik Ehrling and The Museum of HP Calculators therefore assume no responsibility and shall have no liability, consequential or otherwise, of any kind arising from the use of this program material or any part thereof.

This is a version of QPI for the HP42S, based on the program with the same name for the HP48 by Mika Heiskanen and André Schoorl. It finds rational approximations to decimal numbers on the forms nom/den, √(nom/den), nom/den×pi, ln(nom/den), e^{(nom/den)} - choosing the form that seems most suitable.

**Usage:**

Using QPI is simple, just enter a decimal number in the range -9.99999999E9 .. 9.99999999E9 and run QPI. The rational approximation takes a few seconds and the result will be displayed in the upper area of the display. The result is also stored in the alpha register. Note that the stack, all numbered registers and user flags are left unaltered. It is therefore possible to run QPI in the middle of a calculation and then continue after it has run.

Examples:

**Technical notes:**

- The precision used is 10^-9 if in ALL-display mode or the actual accuracy of the chosen display mode if in FIX/SCI/ENG-mode.
- QPI uses continued fraction expansion (lines 177-219) for the rational approximation and then the same set of rules for choosing the form as QPI v4.3 for the HP-48. Generally it prioritises smaller denominators.
- The variables X_QPI, Y_QPI, Z_QPI, T_QPI, L_QPI, R_QPI are used for storing stack and register contents and should be considered reserved variables.
- Screens-shots from Emu42 by Christoph Giesselink

Binary files for emulators:

State-file for Emu42: qpi.e42 Raw binary: qpi.raw Binary for HP-42X: qpi.42x

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