Thread: Calculate the inverse of the matrix using power series.

Calculate the inverse of [1, i/2 ; -1/3, 1] = I - [0, -i/2 ; 1/3, 0] using a power series.


I know that (I - A)^-1 = I + A + A^2 +A^3 but am unsure of how to calculate this without using a computer.

Thanks

There are a few ways to do this. Typically you want to diagonalize any matrix that you're going to raising to powers, but here you can avoid that.

Spoiler:

Thanks. How did you calculate c?

I just did the matrix multiplication (they're 2x2 matricies with half zeros, so that's about than 10 second of work). I didn't know ahead of time it would work out this way. If was only after I did the multiplication that I saw that the power series would thereby algebraically "collapse" the way it did.

As an aside, that's a kinda universal approach I have to math problems. When I'm given something general or abstract, I try to do (or apply it to) a few specific, more concrete examples - or in this case compute a term or two of a series. That often helps me "see" what's going on. Sometimes, like with this problem, I'll find something unexpected that will give me the key to how to solve the problem.