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Re: Newbie question solving lin sys
From: 
John W. Eaton 
Subject: 
Re: Newbie question solving lin sys 
Date: 
Fri, 27 Dec 2002 19:58:34 0600 
On 27Dec2002, John B. Thoo <address@hidden> wrote:
 That works to give me a polygonal curve that connects the 12 points.
 That's good, but how would I plot the polynomial with more grid points
 (smoother)? It works reasonably when I do "format long" and then copy &
 paste the coefficients to plot

 octave:139> gplot [0:11] "22 + 129.316052352359*x 
 330.008191451222*x**2 + 356.743251568416*x**3  211.743557625161*x**4 +
 77.5116682813202*x**5  18.4455900615644*x**6 + 2.90919225949428*x**7 
 0.301909718420775*x**8 + 0.0198178458639753*x**9 
 0.000745701048375786*x**10 + 0.0000122504808365439*x**11"

 but there must be an easier way. :)

 Once again, thanks very much for all your help.
In Octave, a polynomial is represented by its coefficients (arranged
in descending order). For example, a vector C of length N:
p(x) = C(1) x^N + ... + C(N) x + C(N+1).
Try something like
A = [...];
T = [...];
c = flipud (A\T); ## note that you ended up with coefficients in
## the opposite order from what Octave's poly
## functions want...
x = 0:0.1:11;
y = polyval (c, x)
plot (x, y);
Makes a nice plot for me given your original A and T.
jwe

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