/usr/share/octave/packages/signal-1.3.2/besselap.m is in octave-signal 1.3.2-1+b1.
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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 | ## Copyright (C) 2009 Thomas Sailer
##
## This program is free software; you can redistribute it and/or modify it under
## the terms of the GNU General Public License as published by the Free Software
## Foundation; either version 3 of the License, or (at your option) any later
## version.
##
## This program is distributed in the hope that it will be useful, but WITHOUT
## ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
## FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more
## details.
##
## You should have received a copy of the GNU General Public License along with
## this program; if not, see <http://www.gnu.org/licenses/>.
## -*- texinfo -*-
## @deftypefn {Function File} {[@var{zero}, @var{pole}, @var{gain}] =} besselap (@var{n})
## Return bessel analog filter prototype.
##
## References:
##
## http://en.wikipedia.org/wiki/Bessel_polynomials
## @end deftypefn
function [zero, pole, gain] = besselap (n)
if (nargin>1 || nargin<1)
print_usage;
endif
## interpret the input parameters
if (!(length(n)==1 && n == round(n) && n > 0))
error ("besselap: filter order n must be a positive integer");
endif
p0=1;
p1=[1 1];
for nn=2:n;
px=(2*nn-1)*p1;
py=[p0 0 0];
px=prepad(px,max(length(px),length(py)),0);
py=prepad(py,length(px));
p0=p1;
p1=px+py;
endfor;
## p1 now contains the reverse bessel polynomial for n
## scale it by replacing s->s/w0 so that the gain becomes 1
p1=p1.*p1(length(p1)).^((length(p1)-1:-1:0)/(length(p1)-1));
zero=[];
pole=roots(p1);
gain=1;
endfunction
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