[R] Exponentiate a matrix

[Ingmar Visser]

Out of curiosity and possibly for later use: is there an R-function that
does matrix logarithms?
Best, Ingmar

On 9/21/06 8:58 PM, "Dimitrios Rizopoulos"
<Dimitris.Rizopoulos at med.kuleuven.be> wrote:

> Quoting Douglas Bates <bates at stat.wisc.edu>:
> 
>> On 9/21/06, Dimitrios Rizopoulos <Dimitris.Rizopoulos at med.kuleuven.be> wrote:
>>> 
>>> Quoting Duncan Murdoch <murdoch at stats.uwo.ca>:
>>> 
>>>> On 9/21/2006 10:40 AM, Doran, Harold wrote:
>>>>> Suppose I have a square matrix P
>>>>> 
>>>>> P <- matrix(c(.3,.7, .7, .3), ncol=2)
>>>>> 
>>>>> I know that
>>>>> 
>>>>>> P * P
>>>>> 
>>>>> Returns the element by element product, whereas
>>>>> 
>>>>>> P%*%P
>>>>> 
>>>>> Returns the matrix product.
>>>>> 
>>>>> Now, P^2 also returns the element by element product. But, is there a
>>>>> slick way to write
>>>>> 
>>>>> P %*% P %*% P
>>>>> 
>>>>> Obviously, P^3 does not return the result I expect.
>>>> 
>>>> 
>>>> I don't think there's anything built in, but it's easy to write your own:
>>> 
>>> I think there was function mtx.exp() in the Malmig package, but it
>>> seems that this package has been withdrawn from CRAN. An old version
>>> appears to exist in:
>>> 
>>> http://r.meteo.uni.wroc.pl/src/contrib/Descriptions/Malmig.html
>>> 
>>> Best,
>>> Dimitris
>> 
>> Is that function for matrix powers or for the exponential of a matrix
>> (which is what I initally thought that Harold wanted)?  There is a
>> function expm in the Matrix package, patterned on the octave function
>> of the same name, the calculates the matrix exponential for a square
>> matrix.
> 
> this function calculates the n-th power of a matrix, and this is what
> I thought Harold wanted, i.e.,
> 
> P %*% P %*% P %*% P
> 
> should be equal to
> 
> mtx.exp(P, 4)
> 
> 
>>> 
>>> 
>>>> "%^%" <- function(mat, pow) {
>>>>    stopifnot(length(pow) == 1, all.equal(pow, round(pow)), nrow(mat) ==
>>>> ncol(mat))
>>>>    pow <- round(pow)
>>>>    if (pow < 0) {
>>>>      mat <- solve(mat)
>>>>      pow <- abs(pow)
>>>>    }
>>>>    result <- diag(nrow(mat))
>>>>    while (pow > 0) {
>>>>      result <- result %*% mat
>>>>      pow <- pow - 1
>>>>    }
>>>>    result
>>>> }
>>>> 
>>>> Now P %^% 3 will give you the matrix cube.
>>>> 
>>>> Duncan Murdoch
>>>> 
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>>>> 
>>> 
>>> 
>>> 
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>> 
>> ______________________________________________
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>> 
> 
> 
> 
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> ______________________________________________
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-- 
Ingmar Visser
Department of Psychology, University of Amsterdam
Roetersstraat 15, 1018 WB Amsterdam
The Netherlands
http://users.fmg.uva.nl/ivisser/
tel: +31-20-5256735