Add Nyquist filter and ref to pm_remez library.

applications/lofar2/model/pfb_os/dsp_study_erko.txt

@@ -54,6 +54,7 @@
 #      https://www.dsprelated.com/showarticle/184.php
 #   . "Realisering van Digitale Signaalbewerkende Systemen, Toepassingen",
 #      5N290, TUE, P.C.M. Sommen, --> DAC slide 19, 20,
+# * [PM-REMEZ] https://pm-remez.readthedocs.io/en/latest/
 #
 # https://ocw.mit.edu/courses/6-341-discrete-time-signal-processing-fall-2005/
 # Youtube: Guitars 4RL
@@ -259,7 +260,8 @@ c) s-plane and z-plane
       . firls = least squares
       . fircls, fircls1 = constrained ls with pass, stop ripple
     - The Remez or Parks-McClellan algorithm (scipy.signal.remez(), MATLAB
-      firpm)
+      firpm). The pm_remez library for Python in [PM-REMEZ] supports band
+      slope weight function and more taps.
   . MATLAB FIR filters specify order n, so yield n + 1 coefs
   . LS and Remez can do bandpass (= flat), differentiator, hilbert
   . LS and Remez for large Ntaps (> 1000) can fail, but can be achieved
@@ -285,6 +287,11 @@ c) s-plane and z-plane
             = h_lp[k] * cos(k pi), for fc = fs / 2,
               series [1, -1]
 
+- Nyquist filter
+  . A Nyquist filter has 0.5 gain, so -3 dB gain, cutoff frequency at
+    pi / L, or fNyquist / L, and are also known as L-th band filters.
+    The impulse response is zero every L-th sample.
+
 
 5) Finite Impulse Response (FIR) filters
 - FIR filters perform time domain Convolution by summing products of shifted