[R] a really simple question on polynomial multiplication
Dear R people: Is there a way to perform simple polynomial multiplication; that is, something like (x - 3) * (x + 3) = x^2 - 9, please? I looked in poly and polyroot and expression. There used to be a package that had this, maybe? thanks, Erin -- Erin Hodgess Associate Professor Department of Computer and Mathematical Sciences University of Houston - Downtown mailto: [EMAIL PROTECTED] __ R-help@r-project.org mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code.
Re: [R] a really simple question on polynomial multiplication
Dear Erin, Yes. Take a look at page 13 in [1]. HTH, Jorge [1] http://cran.r-project.org/web/packages/Ryacas/vignettes/Ryacas.pdf On Wed, Oct 15, 2008 at 5:44 PM, Erin Hodgess [EMAIL PROTECTED]wrote: Dear R people: Is there a way to perform simple polynomial multiplication; that is, something like (x - 3) * (x + 3) = x^2 - 9, please? I looked in poly and polyroot and expression. There used to be a package that had this, maybe? thanks, Erin -- Erin Hodgess Associate Professor Department of Computer and Mathematical Sciences University of Houston - Downtown mailto: [EMAIL PROTECTED] __ R-help@r-project.org mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code. [[alternative HTML version deleted]] __ R-help@r-project.org mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code.
Re: [R] a really simple question on polynomial multiplication
One way is to use convolution (?convolve): If A(x) = a_p*x^p + ... + a_1*x + a_0 and B(x) = b_q*x^q + ... + b_1*x + b_0 and if C(x) = A(x)*B(x) = c_(p+q)*x^(p+q) + ... + c_0 then c = convolve(a,rev(b),type=open) where c is the vector (c_(p+q),...,c_0), a is (a_p,...,a_0) and b is (b_q,...,b_0). In your case: a - c(1,-3) b - c(1,3) c - convolve(a,rev(b),type=open) c [1] 1.0e+00 -6.49654e-16 -9.0e+00 Note that the coefficient of x is not exactly 0 due to the use of fft to compute convolutions. --- On Thu, 16/10/08, Erin Hodgess [EMAIL PROTECTED] wrote: From: Erin Hodgess [EMAIL PROTECTED] Subject: [R] a really simple question on polynomial multiplication To: R Help r-help@r-project.org Received: Thursday, 16 October, 2008, 8:44 AM Dear R people: Is there a way to perform simple polynomial multiplication; that is, something like (x - 3) * (x + 3) = x^2 - 9, please? I looked in poly and polyroot and expression. There used to be a package that had this, maybe? thanks, Erin -- Erin Hodgess Associate Professor Department of Computer and Mathematical Sciences University of Houston - Downtown mailto: [EMAIL PROTECTED] __ R-help@r-project.org mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code. __ R-help@r-project.org mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code.
Re: [R] a really simple question on polynomial multiplication
On 16/10/2008, at 10:44 AM, Erin Hodgess wrote:
Dear R people:
Is there a way to perform simple polynomial multiplication; that is,
something like
(x - 3) * (x + 3) = x^2 - 9, please?
I looked in poly and polyroot and expression. There used to be a
package that had this, maybe?
The PolynomF package from CRAN will do this. E.g.
install.packages(PolynomF)
library(PolynomF)
a - polynom(-3,1)
b - polynom(3,1)
a*b
The default is to print ``-9 + x^2'' rather than ``x^2 - 9''.
You can of course type
print(a*b,decreasing=TRUE)
to get ``x^2 - 9'', but this is a bit kludgy. It would be nice to have
an options() setting to change the default. You could program that up
yourself of course. (And then send the contributed the improvement to
Bill Venables!)
cheers,
Rolf
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Re: [R] a really simple question on polynomial multiplication
On 16/10/2008, at 2:27 PM, Moshe Olshansky wrote:
Hi Rolf,
Thank you for making me aware of the existence of PolynomF package.
By the way, your solution needs a small modification:
a - polynom(c(-3,1)) and not polynom(-3,1) and similar for b.
Indeed; I typed the code into the email separately from typing it into
my R window. (Bad move!) I did try the code out (honest!) but then
retyped when I sent my email r.t. copying and pasting. (Copying and
pasting on a Mac --- on which I am forced to work --- is fraught with
frustrations.)
cheers,
Rolf
P. S. PolynomF seems to be slickly designed. But then, what do you
expect? Bill Venables wrote it!
R.
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