# fstair

computes pencil column echelon form by qz transformations

## Calling Sequence

    [AE,EE,QE,ZE,blcks,muk,nuk,muk0,nuk0,mnei]=fstair(A,E,Q,Z,stair,rk,tol)

## Arguments

A

  m x n matrix with real entries.

tol

  real positive scalar.

E

  column echelon form matrix

Q

  m x m unitary matrix

Z

  n x n unitary matrix

stair

  vector of indexes (see ereduc)

rk

  integer, estimated rank of the matrix

AE

  m x n matrix with real entries.

EE

  column echelon form matrix

QE

  m x m unitary matrix

ZE

  n x n unitary matrix

nblcks

  is the number of submatrices having full row rank >= 0 detected in matrix `A`.

muk:

  integer array of dimension (n). Contains the column dimensions mu(k) (k=1,...,nblcks) of the submatrices having full column rank in the pencil sE(eps)-A(eps)

nuk:

  integer array of dimension (m+1). Contains the row dimensions nu(k) (k=1,...,nblcks) of the submatrices having full row rank in the pencil sE(eps)-A(eps)

muk0:

  integer array of dimension (n). Contains the column dimensions mu(k) (k=1,...,nblcks) of the submatrices having full column rank in the pencil sE(eps,inf)-A(eps,inf)

nuk:

  integer array of dimension (m+1). Contains the row dimensions nu(k) (k=1,...,nblcks) of the submatrices having full row rank in the pencil sE(eps,inf)-A(eps,inf)

mnei:

  integer array of dimension (4). mnei(1) = row dimension of sE(eps)-A(eps)

## Description

Given a pencil `sE-A` where matrix `E` is in column echelon form the function `fstair` computes according to the wishes of the user a unitary transformed pencil `QE(sEE-AE)ZE` which is more or less similar to the generalized Schur form of the pencil `sE-A`. The function yields also part of the Kronecker structure of the given pencil.

`Q,Z` are the unitary matrices used to compute the pencil where E is in column echelon form (see ereduc)

## See Also

- quaskro
- ereduc