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truncated exponential series over ring of matrices over symbolic ring

asked 2017-12-19 15:33:44 -0600

charleslebarron gravatar image

I want to do the following:

R.<z>=PowerSeriesRing(SR)
i,n,z=var('i,n,z')
f,g=function('f,g')
F=sum(f(i)*z^i,i,1,n)
G=sum(g(i)*z^i,i,1,n)
gamma=matrix([[0,F],[G,0]])

I then want to define a function as the first several terms of the exponential series exp(gamma). Something like

Gamma=sum(gamma^i/factorial(i),i,0,n)

This will return an error like: 'too many values to unpack'

I only want to work with this function formally until I need to specify a value of n and eventually solve for the value f(i), g(i) in terms of some other parameters (yet to be defined in this code). What are some ways to clean this up so I can work with the truncated power series with matrix coefficients?

A possibly simpler but related question is how does one build a function that will sum several powers of a matrix?

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answered 2017-12-19 16:16:06 -0600

tmonteil gravatar image

updated 2017-12-20 04:40:08 -0600

I am not sure about your request, is the following sufficient for you ?

sage: Gamma = lambda n: sum(gamma^i/factorial(i) for i in range(n))
sage: Gamma(2)
[                     1 sum(z^i*f(i), i, 1, n)]
[sum(z^i*g(i), i, 1, n)                      1]

sage: Gamma(3)
[1/2*sum(z^i*f(i), i, 1, n)*sum(z^i*g(i), i, 1, n) + 1                                sum(z^i*f(i), i, 1, n)]
[                               sum(z^i*g(i), i, 1, n) 1/2*sum(z^i*f(i), i, 1, n)*sum(z^i*g(i), i, 1, n) + 1]
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Asked: 2017-12-19 15:33:44 -0600

Seen: 19 times

Last updated: Dec 20 '17