Matrix exponential

How about this:

expm

function(x,pow=2) 
{
xd <- diag((eigen(x)$values^pow))
xm <- eigen(x)$vector %*% xd %*% t(eigen(x)$vector)
return(xm)
}

xa

[,1] [,2]
[1,]    2    1
[2,]    1    3

expm(xa,pow=3)

[,1] [,2]
[1,]   15   20
[2,]   20   35

xa %*% xa %*% xa

[,1] [,2]
[1,]   15   20
[2,]   20   35

From: Federico Calboli <f.calboli at ucl.ac.uk>
To: r-help <r-help at stat.math.ethz.ch>
Subject: [R] matrix exponential: M^0~~

Dear All, 

I would like to ask why the zeroeth power of a matrix gives me a matrix
of ones rather than the identity matrix:

D<-rbind(c(0,0,0),c(0,0,0),c(0,0,0))
D<-as.matrix(D)
D

[,1] [,2] [,3]
[1,]    0    0    0
[2,]    0    0    0
[3,]    0    0    0

D^0

[,1] [,2] [,3]
[1,]    1    1    1
[2,]    1    1    1
[3,]    1    1    1

I would have expected the identity matrix here.

I find the same result with every other square matrix I used.
BTW, I am using R 1.8.1 on Linux Mandrake 9.1

Cheers,

Federico Calboli

=================================

Federico C. F. Calboli

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Dipartimento di Biologia
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f.calboli at ucl.ac.uk