obs_gram observability gramian

## Syntax

                Go = obs_gram(sl)
                Go = obs_gram(A, C)
                Go = obs_gram(A, C, dom)

## Arguments

A,C

  real matrices (of appropriate dimensions)

dom

  string (`"d'` or `"c"` (default value))

sl

  `syslin` list

## Description

Observability gramian of the pair `(A,C)` or linear system `sl` (`syslin` list). `dom` is the domain which can be

"c"

  continuous system (default)

"d"

  discrete system

## Examples

    A=-diag(1:3);C=rand(2,3);
    Go=obs_gram(A,C,'c');     // <=> w=syslin('c',A,[],C); Go=obs_gram(w);
    norm(Go*A+A'*Go+C'*C,1)
    norm(lyap(A,-C'*C,'c')-Go,1)
    A=A/4; Go=obs_gram(A,C,'d');    //discrete time case
    norm(lyap(A,-C'*C,'d')-Go,1)

## See also

- ctr_gram
- obsvss
- obsv_mat
- lyap