Month: April 2014

What is the computational complexity of inverting an nxn matrix? (In 
general, not special cases such as a triangular matrix.)

Gaussian Elimination leads to O(n^3) complexity. The usual way to 
count operations is to count one for each "division" (by a pivot) and 
one for each "multiply-subtract" when you eliminate an entry.

Here's one way of arriving at the O(n^3) result:

   At the beginning, when the first row has length n, it takes n 
   operations to zero out any entry in the first column (one division, 
   and n-1 multiply-subtracts to find the new entries along the row 
   containing that entry. To get the first column of zeroes therefore 
   takes n(n-1) operations.

   In the next column, we need (n-1)(n-2) operations to get the second 
   column zeroed out.

   In the third column, we need (n-2)(n-3) operations.

   The sum of all of these operations is:

         n            n         n      n(n+1)(2n+1)   n(n+1)
        SUM i(i-1) = SUM i^2 - SUM i = ------------ - ------
        i=1          i=1       i=1           6           2

   which goes as O(n^3). To finish the operation count for Gaussian 
   Elimination, you'll need to tally up the operations for the process 
   of back-substitution (you can check that this doesn't affect the 
   leading order of n^3).

You might think that the O(n^3) complexity is optimal, but in fact 
there exists a method (Strassen's method) that requires only 
O(n^log_2(7)) = O(n^2.807...) operations for a completely general 
matrix. Of course, there is a constant C in front of the n^2.807. This 
constant is not small (between 4 and 5), and the programming of 
Strassen's algorithm is so awkward, that often Gaussian Elimination is 
still the preferred method.

Even Strassen's method is not optimal. I believe that the current 
record stands at O(n^2.376), thanks to Don Coppersmith and Shmuel 
Winograd. Here is a Web page that discusses these methods:

   Fast Parallel Matrix Multiplication - Strategies for Practical 
   Hybrid Algorithms - Erik Ehrling
http://www.f.kth.se/~f95-eeh/exjobb/background.html   

These methods exploit the close relation between matrix inversion and 
matrix multiplication (which is also an O(n^3) task at first glance). 

I hope this helps! 

- Doctor Douglas, The Math Forum
http://mathforum.org/dr.math/