ToeplitzInverseUpdate {ltsa}

Inverse of Toeplitz matrix of order n+1 given inverse of order n

Description

Let G be a Toeplitz matrix of order n and with (i,j)-element, r[Abs[i-j]]. So the first row of G may be written (r[0],...,r[n-1]). Suppose the next element in the sequence is r[n]. Then the inverse of the Toeplitz matrix whose first row is (r[0],...,r[n]) may be obtained either using ToeplitzInverseUpdate or directly using TrenchInverse. ToeplitzInverseUpdate is somewhat faster.

Usage

ToeplitzInverseUpdate(GI, r, rnew)

Arguments

GI

inverse of Toeplitz matrix G of order n

r

first row of G , ie r[0],...,r[n-1]

rnew

next element, r[n]

Details

Although this update requires O(n^2) flops, the same as TrenchInverse, it is somewhat faster in practice.

Value

inverse matrix of order n+1

Author(s)

A.I. McLeod

References

Graybill, F.A. (1983). Matrices with Applications in Statistics.

McLeod, A.I., Yu, Hao, Krougly, Zinovi L. (2007). Algorithms for Linear Time Series Analysis, Journal of Statistical Software.

See Also

TrenchInverse

Examples

#In this example we compute the update inverse directly and using ToeplitzInverseUpdate and
#compare the result.
phi<-0.8
sde<-30
n<-30
r<-arima.sim(n=30,list(ar=phi),sd=sde)
r<-phi^(0:(n-1))/(1-phi^2)*sde^2
n1<-25
G<-toeplitz(r[1:n1])
GI<-solve(G) #could also use TrenchInverse
GIupdate<-ToeplitzInverseUpdate(GI,r[1:n1],r[n1+1])
GIdirect<-solve(toeplitz(r[1:(n1+1)]))
ERR<-sum(abs(GIupdate-GIdirect))
ERR