# Rates of convergence

## 1 Speed of convergence

In order to compare perfomance of algorithms we need to define a terminology for different types of convergence. Let r_k = \{\|x_k - x^*\|_2\} be a sequence in \mathbb{R}^n that converges to zero.

### 1.1 Linear convergence

We can define the linear convergence in a two different forms:

\| x_{k+1} - x^* \|_2 \leq Cq^k \quad\text{or} \quad \| x_{k+1} - x^* \|_2 \leq q\| x_k - x^* \|_2,

for all sufficiently large k. Here q \in (0, 1) and 0 < C < \infty. This means that the distance to the solution x^* decreases at each iteration by at least a constant factor bounded away from 1. Note, that sometimes this type of convergence is also called exponential or geometric. We call the q the convergence rate.

Question

Suppose, you have two sequences with linear convergence rates q_1 = 0.1 and q_2 = 0.7, which one is faster?

Example

Let us have the following sequence:

r_k = \dfrac{1}{3^k}

One can immediately conclude, that we have a linear convergence with parameters q = \dfrac{1}{3} and C = 0.

Question

Let us have the following sequence:

r_k = \dfrac{4}{3^k}

Will this sequence be convergent? What is the convergence rate?

### 1.2 Sublinear convergence

If the sequence r_k converges to zero, but does not have linear convergence, the convergence is said to be sublinear. Sometimes we can considet the following class of sublinear convergence:

\| x_{k+1} - x^* \|_2 \leq C k^{q},

where q < 0 and 0 < C < \infty. Note, that sublinear convergence means, that the sequence is converging slower, than any geometric progression.

### 1.3 Superlinear convergence

The convergence is said to be superlinear if:

\| x_{k+1} - x^* \|_2 \leq Cq^{k^2} \qquad \text{or} \qquad \| x_{k+1} - x^* \|_2 \leq C_k\| x_k - x^* \|_2,

where q \in (0, 1) or 0 < C_k < \infty, C_k \to 0. Note, that superlinear convergence is also linear convergence (one can even say, that it is linear convergence with q=0).

\| x_{k+1} - x^* \|_2 \leq C q^{2^k} \qquad \text{or} \qquad \| x_{k+1} - x^* \|_2 \leq C\| x_k - x^* \|^2_2,

where q \in (0, 1) and 0 < C < \infty.

Quasi-Newton methods for unconstrained optimization typically converge superlinearly, whereas Newtonâ€™s method converges quadratically under appropriate assumptions. In contrast, steepest descent algorithms converge only at a linear rate, and when the problem is ill-conditioned the convergence constant q is close to 1.

## 2 How to determine convergence type

### 2.1 Root test

Let \{r_k\}_{k=m}^\infty be a sequence of non-negative numbers, converging to zero, and let

q = \lim_{k \to \infty} \sup_k \; r_k ^{1/k}

• If 0 \leq q \lt 1, then \{r_k\}_{k=m}^\infty has linear convergence with constant q.
• In particular, if q = 0, then \{r_k\}_{k=m}^\infty has superlinear convergence.
• If q = 1, then \{r_k\}_{k=m}^\infty has sublinear convergence.
• The case q \gt 1 is impossible.

### 2.2 Ratio test

Let \{r_k\}_{k=m}^\infty be a sequence of strictly positive numbers converging to zero. Let

q = \lim_{k \to \infty} \dfrac{r_{k+1}}{r_k}

• If there exists q and 0 \leq q \lt 1, then \{r_k\}_{k=m}^\infty has linear convergence with constant q.
• In particular, if q = 0, then \{r_k\}_{k=m}^\infty has superlinear convergence.
• If q does not exist, but q = \lim\limits_{k \to \infty} \sup_k \dfrac{r_{k+1}}{r_k} \lt 1, then \{r_k\}_{k=m}^\infty has linear convergence with a constant not exceeding q.
• If \lim\limits_{k \to \infty} \inf_k \dfrac{r_{k+1}}{r_k} =1, then \{r_k\}_{k=m}^\infty has sublinear convergence.
• The case \lim\limits_{k \to \infty} \inf_k \dfrac{r_{k+1}}{r_k} \gt 1 is impossible.
• In all other cases (i.e., when \lim\limits_{k \to \infty} \inf_k \dfrac{r_{k+1}}{r_k} \lt 1 \leq \lim\limits_{k \to \infty} \sup_k \dfrac{r_{k+1}}{r_k}) we cannot claim anything concrete about the convergence rate \{r_k\}_{k=m}^\infty.
Example

Let us have the following sequence:

r_k = \dfrac{1}{k}

Determine the convergence

Example

Let us have the following sequence:

r_k = \dfrac{1}{k^2}

Determine the convergence

Example

Let us have the following sequence:

r_k = \dfrac{1}{k^q}, q > 1

Determine the convergence

Try to use root test here

Let us have the following sequence:

r_k = \dfrac{1}{k^k}

Determine the convergence