/usr/share/puredata/doc/5.reference/rzero~-help.pd is in puredata-core 0.46.7-3.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
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#X obj 54 85 osc~ 100;
#X msg 69 117 clear;
#X obj 92 520 lop~;
#X text 9 520 see also:;
#X text 402 521 updated for Pd version-0.38;
#X obj 100 459 rzero~;
#X obj 35 483 cpole~;
#X obj 35 459 rpole~;
#X obj 164 459 rzero_rev~;
#X obj 100 483 czero~;
#X obj 164 483 czero_rev~;
#X text 262 459 real;
#X text 261 484 complex;
#X text 33 438 1-pole;
#X text 98 439 1-zero;
#X text 162 438 1-zero \, reversed;
#X text 57 420 summary of raw filters:;
#X msg 71 143 set 1;
#X text 125 143 <-- set internal state;
#X text 134 86 <-- signal to filter;
#X text 137 172 <-- filter coefficient (may be a signal);
#X text 132 195 <-- creation argument initializes filter coefficient
;
#X text 124 116 <-- clear internal state to zero;
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1;
#X obj 214 220 *~;
#X obj 230 197 / 10;
#X text 80 18 Stuff to test it:;
#X obj 76 168 rzero~;
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#X restore 457 416 pd test;
#X text 136 520 etc.: user-friendly filters;
#X obj 52 7 rzero~;
#X text 114 7 real one-zero (non-recursive) filter \, raw;
#X text 8 35 Rzero~ filters an audio signal (left inlet) via a one-zero
real filter \, whose coefficient is controlled by a creation argument
or by an audio signal (right inlet).;
#X obj 53 195 rzero~ 1;
#X text 8 271 where y[n] is the output \, x[n] the input \, and a[n]
the filter coefficient. The filter is always stable.;
#X text 9 307 The transfer function is H(Z) = 1 - aZ^-1.;
#X text 68 250 y[n] = x[n] - a[n] * x[n-1];
#X text 7 330 (Pd also provides a suite of user-friendly filters. This
and other raw filters are provided for situations which the user-friendly
ones can't handle. See Chapter 8 of http://crca.ucsd.edu/~msp/techniques
for an introduction to the necessary theory.);
#X text 14 230 The action of rzero~ is:;
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