**From:** Thomas Lumley (*tlumley@u.washington.edu*)

**Date:** Sat 29 May 2004 - 07:55:48 EST

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Message-id: <Pine.A41.4.58.0405281433000.25454@homer03.u.washington.edu>

W is looking for the vector cross product (a specifically

three-dimensional object important in physics and engineering). The

crossproduct() function provides a matrix product so that crossprod(x,y)

is t(x)%*%y, something completely different.

For the three-dimensional case you could define the cross and dot products

"%x%"<-function(a,b) {c(a[2]*b[3]-a[3]*b[2], -a[1]*b[3]+a[3]*b[1],

a[1]*b[2]-a[2]*b[1])}

"%.%%<-function(a,b) sum(a*b)

It would make sense, of course, to check that the arguments actually were

vectors of length 3.

-thomas

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