/usr/share/puredata/doc/3.audio.examples/buttercoef3.pd is in puredata-doc 0.46.7-3.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 | #N canvas 139 346 714 532 10;
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#X msg 101 108 0.667;
#X msg 82 283 0;
#X obj 517 270 loadbang;
#X obj 528 298 inlet;
#X obj 517 322 f;
#X obj 517 346 expr 1 - 2*$f1;
#X obj 63 79 t b b b f;
#X obj 205 228 * -1;
#X obj 163 228 t f f;
#X obj 63 391 f;
#X obj 30 463 outlet;
#X text 515 237 1 to normalize at Nyquist;
#X text 59 30 characteristic frequency \, 0(DC) to 1(Nyquist);
#X obj 283 470 outlet;
#X obj 439 472 outlet;
#X text 439 494 imag2a;
#X text 283 492 real1;
#X text 374 494 real2;
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#X text 500 495 imag2b;
#X obj 373 470 outlet;
#X text 27 485 normalizer1;
#X obj 173 470 outlet;
#X text 170 492 normalizer2;
#X obj 156 436 expr (($f2-$f1)*($f2-$f1)+$f3*$f3);
#X obj 63 412 t f f;
#X obj 101 176 expr (1 - $f2*$f2) / (1 + $f2*$f2 + 2*$f2*cos($f1))
;
#X obj 163 205 expr 2*$f2*sin($f1) / (1 + $f2*$f2 + 2*$f2*cos($f1))
;
#X obj 82 307 expr (1 - $f2*$f2) / (1 + $f2*$f2 + 2*$f2*cos($f1));
#X obj 522 89 clip 0 1;
#X obj 522 111 expr tan($f1*1.57);
#X obj 101 131 expr $f1*1.5708;
#X text 515 251 0 to normalize at DC;
#X text 119 4 3-pole (or zero) Butterworth filter coefficient calculator
;
#X text 145 109 "theta" in units of pi/2;
#X text 211 138 conjugate pair of pole/zero locations:;
#X text 197 155 real part: (1-r*r)/(1+r*r-2rcos(th));
#X text 245 226 imaginary part: 2rsin(th)/(...);
#X text 270 282 real-valued one \, theta=0;
#X obj 30 439 expr abs($f1-$f2);
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