QR iteration

In the QR iteration, we focus on computing:

$$T_k=Q_k^{H}A{Q}_k$$

where $Q_k$ is the sequence produced by the method of orthogonal iteration.

Let’s consider $T_k$ at the next iteration:

$$T_{k+1}=Q_{k+1}^{H}A{Q}_{k+1}$$

Can we calculate $T_{k+1}$ from $T_k$ without computing $Q_k$?

Step 1:

$Q_{k+1}{R}_{k+1}=A{Q}_k$ from orthogonal iteration.

$$T_k=Q_k^H(A{Q}_k)=Q_k^H{Q}_{k+1}{R}_{k+1}\stackrel{def}{=}{U}_{k+1}{R}_{k+1}$$

where we define $U_{k+1}\stackrel{def}{=}{Q}_k^H{Q}_{k+1}$.

We can interpret $Q_k^H{Q}_{k+1}$ as being a small rotation. It corresponds to an increment in $Q_k$ as $k$ increases and $Q_k$ converges to the Schur orthogonal transformation.

$U_{k+1}$ and $R_{k+1}$ can be computed using a QR decomposition without the help of the orthogonal iteration. $Q_k$ or $Q_{k+1}$ are not needed.

Step 2:

This is the definition for step $k+1$:

$$T_{k+1}=Q_{k+1}^{H}A{Q}_{k+1}$$

and recall that $T_k=Q_k^{H}A{Q}_k$. This leads to:

$$T_{k+1}=Q_{k+1}^H{Q}_k{T}_k{Q}_k^H{Q}_{k+1}.$$

Note $Q_k$ must be square for this to be true. Recall the definition from the previous step:

$$U_{k+1}=Q_k^H{Q}_{k+1}$$

We find that $T_k$ is unitarily similar to $T_{k+1}$:

$$T_{k+1}=U_{k+1}^H{T}_k{U}_{k+1}$$

But recall from the step above that

$$T_k=U_{k+1}{R}_{k+1}$$

So $U_{k+1}^H{T}_k=R_{k+1},$ and

$$T_{k+1}=R_{k+1}{U}_{k+1}$$

This is the basis for the method of QR iteration.

Summary of the key steps

• Step 1: $T_k=U_{k+1}{R}_{k+1}$: QR decomposition.
• Step 2: $T_{k+1}=R_{k+1}{U}_{k+1}$: apply the orthogonal transformation to the right of $R_{k+1}$.

We just switch the order of the terms!

We can compute $T_{k+1}$ from $T_k$ without computing any of the $Q_k$.

Although these steps are different from orthogonal iteration, this is still the same iteration. For example, the sequence of $R_k$ matrices and the convergence is the same for both methods.

We can also connect the sequence of $U_k$ matrices to the orthogonal iteration. Since

$$U_{k+1}=Q_k^H{Q}_{k+1}$$

we can prove by induction that

$$Q_k=U_1\cdots U_k$$

if we start from $Q_0=I$. The product of the $U_i$ in the QR iteration is equal to $Q_k$ in the orthogonal iteration, and again converges to the orthogonal matrix $Q$ from the Schur decomposition.

QR iteration algorithm

Tk = A
while not_converged:
    Uk, Rk = qr(Tk)
    Tk = Rk * Uk

Here are the first few iterations:

• $Q_1{R}_1=A$
• $Q_2{R}_2=R_1{Q}_1$
• $Q_3{R}_3=R_2{Q}_2$
• $Q_4{R}_4=R_3{Q}_3$
• …