I am here (MidiMagic).
The information is as accurate as I could obtain, excepting that I did not use the trig versions. I put the trig info in another article
How to do Calculations for Quadraphonics on the site. The rounded (not truncated) values are for setting matrix parameters on analog controls (such as the mixer I use and my surround controller). The average user does not have a scientific calculator at hand.
The "QS" equations on the site (as of yesterday) are the Scheiber equations. I put QS into the same position in the chart as the Scheiber equations because the encoding positions are the same and the same decoders work. The article was originally from the operation manual for my SSC-10 surround control unit. It treats Scheiber and QS identically. I was going to add the QS equations to the web page and forgot I hadn't done it. I will do it as soon as possible.
The BMX equations are derived from the equations in "Discrete-Matrix Multichannel Stereo" D.H. Cooper & T. Shiga, Journal of the Audio Engineering Society 06/1972 V 20 pp. 346-360. The article does not directly give 4-corners coefficients. If you have different BMX equations, I would like to see them.
BMX is not QS. It is the baseband of UD-4. But Denon also had an early system called QM with regular-matrix properties. Don't confuse them. QS decoders play a faux version of BMX because sounds in BMX in between mics comes out at the QS and RM front.
My info about UMX, BMX, and UD-4 are from "A Geometric Model for Two Channel Four Speaker Matrix Stereo Systems" Michael Gerzon, Journal of the Audio Engineering Society 03/1975 V 23 pp. 98-106. Notice how BMX and UMX are recorded at right angles to QS in Gerzon's diagrams.
My CD-4 writings come from experience. I have NEVER heard a CD-4 recording that worked properly. But at the time I had CD-4 equipment (that was given to me by my employer as an evaluation sample), I was a college student and bought used records. I was using a PE 2038 with low capacity cables, an AT-12 cartridge, and a JVC demodulator. But I never heard a demonstration of CD-4 that worked right either. I always heard at least one of these:
1. The loud snapping from dust on the record
2. The demodulator kicking in and out (probably improper cartridge installation)
3. The background hiss from having played the same record over and over
I decided early on that CD-4 was not worth the effort.
Your mention of Trump is irrelevant.