<p>In mathematics a <em>fixed point</em> is, in general, in  a mapping of a space in itself, a point that corresponds to itself.</p>
<p>In particular, fixed points for functions <span class="math-container">$f(x)$</span> are value of the variables of the function such that</p>
<p><span class="math-container">$$f(x)=x.$$</span>
One can speak of fixed point for function from <span class="math-container">$\mathbb{R}^N$</span> to  <span class="math-container">$\mathbb{R}^N$</span>  (or, in general, from a metric space in itself), or for functions between topological spaces.</p>
<p>Fixed point theorems are a family of theorems about the existence of fixed points of a function. Well known theorems are the <strong>Contraction theorem</strong>, for functions from a metric space in itself , and <strong>Brouwer fixed point theorem</strong> in topology.</p>
<p>In <strong>discrete dynamic systems</strong>, which from  a mathematical point of view are sequences by recurrence, we speak also of fixed points. They are points of <strong>equilibrium</strong>, in the sense that the dynamic system, once has arrived at that point, remains here, it is a 'state of rest' of the system.</p>
<p>And it can be seen that they are fixed point of a function, reconnecting to the definition above.</p>
<p>Actually, a  dynamic discret system, given a function <span class="math-container">$f$</span> from <span class="math-container">$\mathbb{R}$</span> to <span class="math-container">$\mathbb{R}$$^{(1)}$</span>, is  recursively described as follows:</p>
<p><span class="math-container">$$\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;x_n=f(x_{n-1}) \;\,\; \;\,\;n=1, 2, ......\; \;\,\;\; \;\,\;(1)$$</span>
<span class="math-container">$$x_0=p \;\;\;\;\;p\in \mathbb{R}$$</span></p>
<p>where <span class="math-container">$p$</span> is t<em>he initial condition.</em></p>
<p>A <em>fixed point of the system</em>, relative to <span class="math-container">$p$</span>, is a fixed point of <span class="math-container">$f$</span>,  that is a point <span class="math-container">$x$</span> where</p>
<p><span class="math-container">$$x_n=x_{n-1}=f(x_{n-1}).$$</span></p>
<p>A fixed poind is also called a <em>stationary point</em>.</p>
<p>And it is a point of equilibrium of the system, in the sense that the system, once has reached the fixed point <span class="math-container">$x_n$</span>,  gives always the same value <span class="math-container">$x_n$</span>, it remains here forever (of course, if there isn't an exogenous disturbing factor that leads it away from equilibrium).</p>
<p>And, similarly, numerical analysis speaks of 'fixed point iteration', as a method to find a fixed point of a function. The concept is analogous to that of discrete dinamical systems.</p>
<p>I quote from Wikipedia &quot;Fixed points&quot;:</p>
<blockquote>
<p>&quot;In <strong>numerical analysis</strong>, fixed-point iteration is a method of
computing fixed points of a function. Specifically, given a function
<span class="math-container">$f$</span> with the same domain and codomain, a point <span class="math-container">$x_{0}$</span> in the domain
of <span class="math-container">$f$</span>, the fixed point iteration is
<span class="math-container">$$x_n=x_{n-1}=f(x_{n-1});\,\; \;\,\;\,\; \;\,\;$$</span>
which  gives rise to sequence <span class="math-container">$x_ 0, x_ 1, x_ 2, …$</span>
of iterated function applications <span class="math-container">$x_ 0 , f (x_0),$</span>
<span class="math-container">$f (f( x_0 )) , … $</span>
which can to converge to a point x.
Points that come back to the same value after a finite number of
iterations of the function are called periodic points. A fixed point
is a periodic point with period equal to one.</p>
</blockquote>
<p>Unfortunately, I couldn't look at the paper you linked, because it is not of free access.</p>
<p>But you said that</p>
<blockquote>
<p>the default initialization is such that the code reaches the fixed
point for the baseline economy after one iteration.</p>
</blockquote>
<p>so I suppose that concept is the same as in numerical analysis.</p>
<p>But apart from computation in numerical analysis, the key conceptual idea  is that in a discrete dynamical system a fixed point is a point of equilibrium, in the sense described above.</p>
<hr />
<p>(1) Of course, there could be also dynamic systems in several variables.</p>

CC BY-SA 4.0
2022-11-07T10:50:38.190Z
This doesn't really answer the question, at least not explicitly.
BrsG
CC BY-SA 4.0
2022-11-07T10:53:45.143Z
What's really the question? Is 'what is the fixed point problem'.And what is its relationship with the concept of equilibrium. It is a general concept in mathematics. For a more specific answer one has to  read the quoted paper.
BakerStreet
CC BY-SA 4.0
2022-11-07T11:47:21.907Z
The question “Why does the fixed points matter in dynamic optimization?” makes no much sense, too general to be answered. There isn’t a specific thing as ‘fixed points in dynamic optimization’, it makes no sense.  Dynamic optimization is an enormous field with subjects as calculus of variations, optimal control, dynamic programming. I could only guess that the OP referred to numerical methods to find fixed point., maybe in dynamic programming.  But the meaning of fixed points is the same everywhere.
BakerStreet
CC BY-SA 4.0
2022-11-05T16:01:06.397Z
2022-11-06T11:48:57.397Z
2022-11-06T11:48:57.397Z
BakerStreet
<p>At the end of the day, you are looking for an equilibrium. An equilibrium is essentially a fixed point.  In equilibrium all agents maximize their payoffs, e.g., optimize (dynamically or statically) their strategies.  Namely, an intersection of those optimal strategies is an equilibrium, or a fixed point.</p>

CC BY-SA 4.0
2022-02-08T10:15:39.903Z
"*That's trivial, you know. That's just a fixed point theorem.*" - [von Neumann on Nash's existence theorem](https://mattbaker.blog/2015/05/26/john-nash-and-the-theory-of-games/#:~:text=Nash%20started%20to%20describe%20the,just%20a%20fixed%20point%20theorem.%E2%80%9D).
Giskard
CC BY-SA 4.0
2022-02-08T14:38:23.167Z
Thanks for your comments. Additionally I am wondering 'fixed point problem' is a problem that really matters in finding equilibrium. It is the first time to be mentioned a fixed point 'problem'.
user14261785
CC BY-SA 4.0
2022-02-08T09:44:22.460Z
2022-02-08T09:44:22.460Z
Green.H
<p>I understand what the fixed point is, but don't understand what the fixed point 'problem'.
Is it resolved by 'fixed point iteration'?
I am reading a paper, and the paper mentions that the default initialization is such that the code reaches the fixed point for the baseline economy after one iteration. Why does the fixed point matter in dynamic optimization?</p>

CC BY-SA 4.0
2022-02-07T22:51:42.320Z
Please link to the cryptic paper?
Giskard
CC BY-SA 4.0
2022-02-08T03:00:06.920Z
Oh, yes I am reading 'Business Cycle dynamics under rational inattention (MW, 2015)' and looking at the code. https://academic.oup.com/restud/article-abstract/82/4/1502/2607355
user14261785
<dynamic-optimization>
What is the 'fixed point problem'?
user14261785
# What is the 'fixed point problem'?

(Asked by: username_0 on 2022-02-07)

I understand what the fixed point is, but don't understand what the fixed point 'problem'.
Is it resolved by 'fixed point iteration'?
I am reading a paper, and the paper mentions that the default initialization is such that the code reaches the fixed point for the baseline economy after one iteration. Why does the fixed point matter in dynamic optimization?

**Tags:** dynamic-optimization
> **Comments:**
> **username_1**: Please link to the cryptic paper?  
> **username_0**: Oh, yes I am reading 'Business Cycle dynamics under rational inattention (MW, 2015)' and looking at the code. https://academic.oup.com/restud/article-abstract/82/4/1502/2607355  

---

## 2 answers

### Answer by username_2 on 2022-11-05. Score: 6

In mathematics a *fixed point* is, in general, in a mapping of a space in itself, a point that corresponds to itself.

In particular, fixed points for functions $f(x)$ are value of the variables of the function such that

$$f(x)=x.$$
One can speak of fixed point for function from $\mathbb{R}^N$ to $\mathbb{R}^N$ (or, in general, from a metric space in itself), or for functions between topological spaces.

Fixed point theorems are a family of theorems about the existence of fixed points of a function. Well known theorems are the **Contraction theorem**, for functions from a metric space in itself , and **Brouwer fixed point theorem** in topology.

In **discrete dynamic systems**, which from a mathematical point of view are sequences by recurrence, we speak also of fixed points. They are points of **equilibrium**, in the sense that the dynamic system, once has arrived at that point, remains here, it is a 'state of rest' of the system.

And it can be seen that they are fixed point of a function, reconnecting to the definition above.

Actually, a dynamic discret system, given a function $f$ from $\mathbb{R}$ to $\mathbb{R}$$^{(1)}$, is recursively described as follows:

$$\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;x\_n=f(x\_{n-1}) \;\,\; \;\,\;n=1, 2, ......\; \;\,\;\; \;\,\;(1)$$
$$x\_0=p \;\;\;\;\;p\in \mathbb{R}$$

where $p$ is t*he initial condition.*

A *fixed point of the system*, relative to $p$, is a fixed point of $f$, that is a point $x$ where

$$x\_n=x\_{n-1}=f(x\_{n-1}).$$

A fixed poind is also called a *stationary point*.

And it is a point of equilibrium of the system, in the sense that the system, once has reached the fixed point $x\_n$, gives always the same value $x\_n$, it remains here forever (of course, if there isn't an exogenous disturbing factor that leads it away from equilibrium).

And, similarly, numerical analysis speaks of 'fixed point iteration', as a method to find a fixed point of a function. The concept is analogous to that of discrete dinamical systems.

I quote from Wikipedia "Fixed points":

> "In **numerical analysis**, fixed-point iteration is a method of
> computing fixed points of a function. Specifically, given a function
> $f$ with the same domain and codomain, a point $x\_{0}$ in the domain
> of $f$, the fixed point iteration is
> $$x\_n=x\_{n-1}=f(x\_{n-1});\,\; \;\,\;\,\; \;\,\;$$
> which gives rise to sequence $x\_ 0, x\_ 1, x\_ 2, …$
> of iterated function applications $x\_ 0 , f (x\_0),$
> $f (f( x\_0 )) , … $
> which can to converge to a point x.
> Points that come back to the same value after a finite number of
> iterations of the function are called periodic points. A fixed point
> is a periodic point with period equal to one.

Unfortunately, I couldn't look at the paper you linked, because it is not of free access.

But you said that

> the default initialization is such that the code reaches the fixed
> point for the baseline economy after one iteration.

so I suppose that concept is the same as in numerical analysis.

But apart from computation in numerical analysis, the key conceptual idea is that in a discrete dynamical system a fixed point is a point of equilibrium, in the sense described above.

---

(1) Of course, there could be also dynamic systems in several variables.

> **Comments:**
> **username_3**: This doesn't really answer the question, at least not explicitly.  
> **username_2**: What's really the question? Is 'what is the fixed point problem'.And what is its relationship with the concept of equilibrium. It is a general concept in mathematics. For a more specific answer one has to  read the quoted paper.  
> **username_2**: The question “Why does the fixed points matter in dynamic optimization?” makes no much sense, too general to be answered. There isn’t a specific thing as ‘fixed points in dynamic optimization’, it makes no sense.  Dynamic optimization is an enormous field with subjects as calculus of variations, optimal control, dynamic programming. I could only guess that the OP referred to numerical methods to find fixed point., maybe in dynamic programming.  But the meaning of fixed points is the same everywhere.  

---

### Answer by username_4 on 2022-02-08. Score: 0

At the end of the day, you are looking for an equilibrium. An equilibrium is essentially a fixed point. In equilibrium all agents maximize their payoffs, e.g., optimize (dynamically or statically) their strategies. Namely, an intersection of those optimal strategies is an equilibrium, or a fixed point.

> **Comments:**
> **username_1**: "*That's trivial, you know. That's just a fixed point theorem.*" - [von Neumann on Nash's existence theorem](https://mattbaker.blog/2015/05/26/john-nash-and-the-theory-of-games/#:~:text=Nash%20started%20to%20describe%20the,just%20a%20fixed%20point%20theorem.%E2%80%9D).  
> **username_0**: Thanks for your comments. Additionally I am wondering 'fixed point problem' is a problem that really matters in finding equilibrium. It is the first time to be mentioned a fixed point 'problem'.  

---

