The name "functional equation" is used for problems where the goal is to find all functions satisfying the given equation (and maybe some other conditions). So in this case, solving the equation means finding all functions fulfilling the equation. (This is different from the more common use of the ...

learn more… | top users | synonyms

4
votes
1answer
64 views

Is distributivity sufficient to define composition?

Function Composition has the property of distributivity: $$(f\star g)\circ h = (f\circ h)\star(g\circ h)\;\forall f,g,\star \in\{+,-,\times,\div\}$$ I was wondering if these properties uniquely ...
2
votes
2answers
105 views

When can we take that $f(1)=1$?

I have been doing some functional equations and in some of them they just say " WLOG let $f(1)=1$ ", but I don't get why they can do that... Can someone please help me? I can't find the example of ...
8
votes
2answers
359 views

Solve the following functional equation $f(xf(y))+f(yf(x))=2xy$

Find all function $f:\mathbb{R}\rightarrow \mathbb{R}$ so that $f(xf(y))+f(yf(x)=2xy$. By putting $x=y=0$ we get $f(0)=0$ and by putting $x=y=1$ we get $f(f(1))=1$. Let $y=f(1)\Rightarrow ...
6
votes
0answers
90 views

Is there a function $f(x)$ such that $f(f(x))=e^x$? [duplicate]

Is there a function which would be a "functional square root" of the exponential function? I.e., function $f(x)$ such that: \begin{equation} f(f(x))\equiv f^2(x)=e^x \end{equation} If not, what is ...
0
votes
0answers
79 views

Existance of solution to integral Fredholm equation

I am a bit confused with the existence/non existence of a solution to the following equation: $$\int_{-T}^{T} R(t,\tau) x(\tau) d\tau = y(t), \forall t\in [-T,T]$$ where $y(t)=C$ (a constant ...
2
votes
0answers
48 views

All $f$ such that $(\exists k)(f^\prime(x) = f(x+k))$ [duplicate]

I was wondering if there is a general way to solve the functional equation $$(\exists k)(f^\prime(x) = f(x+k))$$ I know that this is true for certain functions: $$(e^{cx})^\prime = e^{c(x+\frac{\ln ...
2
votes
0answers
41 views

Don't understand why this generating function needs to be taken to the power $-1$

I'm given the following recursive formula for a sequence: $u_{n+1}=u_n+\sum_{k=1}^{n-1}u_ku_{n-k}\ (n\ge1),\ u_0=0,\ u_1=1$ Since $u_0=0$ we can rewrite: $u_{n+1}=u_n+\sum_{k=0}^{n}u_ku_{n-k},\ ...
2
votes
2answers
94 views

Cauchy functional equation and the implicit function theorem

The problem statement: Suppose $f:\mathbb{R} \to \mathbb{R}$ is continuously differentiable, $f'(x)$ is strictly increasing, with $\lim_{x \to -\infty}f'(x) = -\infty$, $\lim_{x \to \infty}f'(x) = ...
4
votes
3answers
121 views

Solve $\sin \frac{\alpha - \beta}2 + \sin \frac{\alpha - \gamma}2 + \sin \frac{3\alpha}2 =\frac 3 2$

Solve the following trigonometric eqation where $\alpha, \beta, \gamma$ are angles in a triangle ($\alpha + \beta + \gamma = 180$): $$\sin \frac{\alpha - \beta}2 + \sin \frac{\alpha - \gamma}2 + \sin ...
5
votes
0answers
193 views

Infinite Double Exponential Sum, with Functional Equation $g(x) = g(\sqrt{x})$

What is a closed form for $$ \lim_{n\to-\infty}\sum_{i=n}^{\infty}\frac{x^{2^i}(x^{2^i}-1)}{(x^{2^{i+2}}+1)} $$ The series has the form: $$... \frac{x^{\frac{1}{4}}(x^{\frac{1}{4}}-1)}{x+1} + ...
1
vote
1answer
142 views

Functions such that $f(f(n))=n+2015$ [duplicate]

Is there a function $f:\mathbb N \to \mathbb N$ such that $\forall n \in \mathbb N, f(f(n))=n+2015$ ? Here's what I've done: Assuming such a function exists, ...
3
votes
1answer
74 views

$(x-y)f(x+y)-(x+y)f(x-y)=4xy(x^2-y^2)$ and $f(1)=2$

Let $f$ be a differentiable function such that $(x-y)f(x+y)-(x+y)f(x-y)=4xy(x^2-y^2)$ and $f(1)=2$. Then area enclosed by $$\frac{|f(x)-x|^{1/3}}{17}+\frac{|f(y)-y|^{1/3}}{2}\le\frac14$$ I rearranged ...
0
votes
1answer
25 views

$(r^2-s^2)^2-(5\cdot\min\{r,s\})=2015$. Find all positive integer solution of this equation.

I know the $\min\{x,y\}$ means the minimum value of $x$ and $y$. and it can be expressed as, $\min\{x,y\}= \frac12\left( x+y-\sqrt{(x-y)^2}\right)$
-3
votes
2answers
207 views

Functional equation $f(ax)=bf(x)$ [duplicate]

What are all the solutions to the functional equation $f(ax)=bf(x)$, where $a,b>0$, and $f$ is continuous, strictly monotone and increasing, and $x$ ranges over the reals? references? proof? ...
2
votes
1answer
211 views

How to prove that Riemann zeta function is zero for negative even numbers? [duplicate]

Can anyone please explain to me how to prove that Riemann zeta function is 0 for all negative even numbers. In many references , they have just given the statement without any proof. Any explanation ...
2
votes
0answers
76 views

Is there a way to graphically show that a solution is the minimum or stationary solution to a functional?

I'm looking for the functional analogue to the visual representations of function optimization you most commonly see. To illustrate, if we have some function: $$ f(x) = (x-1)^2+1 $$ We can look at ...
2
votes
1answer
57 views

General Solution to Almost Riccati Like Equation

Consider the differential equation $$ y' = a_0(x) + a_1(x)y + a_2(x)\frac{1}{y}$$ I am attempting to find the general solution to this. One thing I can note is that the entire equation can be ...
5
votes
1answer
46 views

Can $f(x+1) = f(x)^{\ln(x)}$ be expressed as integral transform $\int g(x,t) dt $?

Let $x$ be a real number. Can some real-analytic function $f$ that satisfies for $x>3$ :$f(x+1) = f(x)^{\ln(x)}$ be expressed by standard functions as an integral transform : $$f(x) = ...
4
votes
0answers
135 views

Show that f is periodic if $f(x+a)+f(x+b)=\frac{f(2x)}{2}$?

Suppose $a$ and $b$ are distinct real numbers and $f$ is a continuous real function such that $\frac{f(x)}{x^2}$ goes to 0 when $x$ goes to infinity or minus infinity. Suppose that$ ...
0
votes
1answer
40 views

Difficult Functional Equation Problem, Non-Standard Type

Find all functions, $f:\mathbb{N} \to \mathbb{N}$, for which $f(1) = 1, f(2n) < 6f(n)$, and $$3f(n)f(2n+1) = f(2n)(3f(n)+1).$$ My first approach is to try to play around and set values equal to ...
2
votes
2answers
69 views

A monotonic multiplicative integer functional equation.

Let $ f:\mathbb N \to \mathbb N $ be such that $ f (x)> f (y)$ if $x> y$. $ f (xy)=f (x) f (y) $. $ f (3) \geq 7$. Find the smallest value of $ f (3) $ My attempt:if we can define the ...
3
votes
5answers
336 views

How are Zeta function values calculated from within the Critical Strip?

We note that for $Re(s) > 1$ $$ \zeta(s) = \sum_{i=1}^{\infty}\frac{1}{i^s} $$ Furthermore $$\zeta(s) = 2^s \pi^{s-1} \sin \left(\frac{\pi s}{2} \right) \Gamma(1-s) \zeta(1-s)$$ Allows us to ...
2
votes
1answer
68 views

General Solution to functional equation

I was wondering how to derive the solution to $$ \frac{f(x + (1-2x)) - f(x)}{1-2x} = f(x)$$ Which can be simplified to $$\frac{f(1-x) - f(x)}{1-2x} = f(x)$$ One idea is as follows. Consider the ...
5
votes
1answer
192 views

Prove that a $1$-periodic function $\phi$ is constant if $\phi(\frac{x}{2}) \phi(\frac{x+1}{2})$ is a constant multiple of $\phi$

For $0 < x< \infty$, let $\phi (x)$ be positive and continuously twice differentiable satisfying: (a) $\phi (x+1) = \phi (x)$ (b) $\phi(\frac{x}{2}) \phi(\frac{x+1}{2}) = d\phi(x),$ where ...
5
votes
0answers
118 views

Convergence of the solution of Volterra integral equation with convergent kernel.

Consider the following Volterra integral equation $$ g(t) = \int_0^t K_n(t,s)w_n(s) ds $$ where $g(t)$ and $K_n(t,s)$ are known(continuous) and $K_n(t,s)\geq K_{n+1}(t,s)$ for all $t,s$. Moreover, ...
4
votes
2answers
113 views

Solve functional equation $(x+y)(f(x)-f(y))=f(x^2)-f(y^2)$

Find all real functions $f\colon \mathbb{R}\rightarrow \mathbb{R}$ so that $(x+y)(f(x)-f(y))=f(x^2)-f(y^2)$. Can someone at least find the value of $f(1)$ if it is possible, it would help me.
7
votes
2answers
369 views

Defining sine and cosine

We know the following are true about sine and cosine (and that they can be proven geometrically): $\sin(a+b)=\sin(a)\cos(b)+\sin(b)\cos(a)$ $\cos(a+b)=\cos(a)\cos(b)-\sin(a)\sin(b)$ ...
5
votes
1answer
82 views

Evaluation of $\int_{-\infty}^{\infty}\operatorname{e}^{-\mu x^2}f(\nu x)\operatorname{d\!}x$ for $\mu>0$

I'm trying to evaluate the integral $$ \Psi(\mu,\nu)=\int_{-\infty}^{\infty}\operatorname{e}^{-\mu x^2}f(\nu x)\operatorname{d}\!x\qquad(\text{for}\; \mu>0)\tag 1 $$ where $\nu\in\Bbb R$ and $f$ ...
1
vote
2answers
45 views

Is there any standard terminology for this property?

Let $f$ be a map whose domain is $X$. If $f$ satisfies the property that for all $x\in X$, $$f(f(x))=f(x)\text{,}$$ is there any standard name for such a function? Not sure if "projection" is the ...
1
vote
1answer
33 views

Additive & Multiplicative Link

Is there an isolative property about the following multivariable equation: $$f_1(x)g_1(y)=f_2(x)+g_2(y)$$ That is, is this equation rearrangable such that it may be put in the form ...
0
votes
1answer
58 views

Difficult Functional Derivative

I've been working on this problem set for a little bit now and I've finally made it to the last question. I'm now left with this monster and I don't quite understand where to proceed. All the ...
1
vote
0answers
43 views

Commutivity of second functional derivatives

In multivariable calculus we learn that partial derivatives are commutative: i.e. $\partial _{xy}F=\partial_{yx}F$ When dealing with multiple functional derivatives this doesn't seem to be the case. ...
4
votes
0answers
126 views

Symmetry and the zeta function

It is an incontestable fact that symmetry is one of, if not the, most powerful tools a mathematician can have at his or her disposal. What I want to know, is if there are any good reasons as to why it ...
0
votes
1answer
49 views

Solution to functional equation and Cauchy

I want to show that the only continuos solutions to the functional equation $g(x)+g(y)=g(\sqrt{x^2+y^2})$ is $g(x)=cx^2$ where c is a constant i.e. c=g(1). I think that I maybe could rewrite the ...
0
votes
0answers
44 views

Conjecture about $A f(x) = f(g(x)) + f(h(x))$

Let a given real $A$ satisfy $0 < A < 2$. Conjecture : For any real entire nonconstant $f(x)$ there exist real entire $g(x)$ and $h(x)$ such that $A f(x) = f(g(x)) + f(h(x))$ or $ A f(x) ...
0
votes
1answer
36 views

Problem involving distances on a parametrically defined curve

SO the problem is stated below: A curve is defined paramterically by: $$x(t)=a\cosh t$$ and $$y(t)=b\sinh t$$ where a and b are positive constants and $-\infty < t < \infty$ The expression ...
2
votes
1answer
41 views

Newton method why the error is proportional to the square for the error of the last one?

We have learned that the Newton method is used to solve different equations. As I know, this method is iterative, which means that using an estimate point and using a loop, we can get closer and ...
9
votes
4answers
177 views

Solve $f\left(\frac{x-3}{x+1}\right)+f\left(\frac{x+3}{x+1}\right)=x$ $\forall x\neq -1$

Given function $y=f(x)$ such that $$f\left(\frac{x-3}{x+1}\right)+f\left(\frac{x+3}{x+1}\right)=x \quad\forall x\neq -1$$ find $f(x)$ and $f(2007)$.
4
votes
2answers
109 views

Proving the shortest function connecting two points is a straight line WITHOUT assuming the Euler-Lagrange equation

For learning purposes, I'm trying to prove that the shortest function passing through the two points $(x_1, y_1)$, $(x_2, y_2)$ is a straight line, without using the Euler-Lagrange equation. My ...
2
votes
1answer
72 views

Solve the functional equation $4f(x)=f(2x)$

Solve the functional equation $4f(x)=f(2x)$. As for now I know that one solution is $f(x)=cx^2$, where c is a constant value.
0
votes
2answers
296 views

What is the minimum degree for a polynomial to pass through points with defined slopes [duplicate]

I'm having some difficulty solving this problem. The information I have is the following: What is the minimum degree for a polynomial for it to pass through points $A(x_1,y_1)$ and $B(x_2,y_2)$ with ...
2
votes
2answers
180 views

Find all real real functions that satisfy the following eqation $f(x^2)+f(2y^2)=[f(x+y)+f(y)][f(x-y)+f(y)]$

Find all real functions $f:\Bbb R\rightarrow\Bbb R$ so that $f(x^2)+f(2y^2)=[f(x+y)+f(y)][f(x-y)+f(y)]$, for all real numbers $x$ and $y$. $f(x)=x^2$ is the only solution I think. So far I have got: ...
0
votes
1answer
39 views

Number of possible solutions to equation

I am trying to solve $$x+y+z = 32$$ Where $x$, $y$, and $z$ are positive integers I believe the answer is: $C_{2}^{31}=465$ but I am not sure why. Can someone please explain?
0
votes
1answer
38 views

How do find the distribution of a R.V. from this functional/differential equation?

Assume that $X$ is some R.V. with domain $[0,1]$. A function $U:[0,1]\to[0,1]$ is defined as follows: $$U(x)=x\cdot F_X(x)+(1-x)(1-F_X(x)) = 2x\cdot F_X(x)- x - F_X(x) + 1$$ Where $F_X(x)=\int_0^x ...
7
votes
2answers
154 views

Function such that $f(x) f(\pi/2 - x) = 1$

I'm looking for functions that are smooth ($C^\infty$) between $0 < x < \pi/2$ that satisfy the equation $$f(x)\, f(\pi/2-x) = 1$$ on the inteverval $0<x<\pi/2$. I know that the constant ...
3
votes
1answer
65 views

Solve functional equation $f(a)f(b)=\frac{1}{2}f(a+b)+\frac{1}{2} f(max(|b-a|,a))$

Solve functional equation $$f(a)f(b)=\frac{1}{2}f(a+b)+\frac{1}{2} f(max(|b-a|,a)),(*)$$ where the following conditions are satisfied: 1)$f:\mathbb{R}_{+}\cup \{0\}\rightarrow [0,1] ,$ 2)$f(0)=1,$ ...
2
votes
1answer
46 views

Soultion of a particular functional differential equation

I need to solve the following functional differential equation: $(4\mu-\lambda r - \lambda x+\lambda)f'(x) = \lambda( f(x) - f(1-x-r))$, where $x\in (0, 1-r)$, $r\in (\frac{1}{2}, 1)$ Or, a more ...
-1
votes
2answers
52 views

Solve the functional equation $f(x+a+f(y))=f(f(x))+a+y$ [closed]

So let $a$ be a real number. Find all functions $f:\mathbb{R}\rightarrow \mathbb{R}$ so that $f(x+a+f(y))=f(f(x))+a+y$, for all real $x,y$.
0
votes
1answer
78 views

Cauchy's Functional Equation

Consider Cauchy's Functional Equation $$\phi(t+s)=\phi(t)+\phi(s).$$ Can we say that any right continuous with left limits (cadlag) solution is Borel measurable? Obviously continuous solutions are ...
4
votes
2answers
109 views

Solve the functional equation $2f(x)=f(ax)$ for some $a$.

I am trying to solve the following functional equation, and could use some help.$$ 2f(x)=f(ax)$$ For some $a\in\mathbb{R}$. By repeated adding $2f(x)$ together we notice that $$2nf(x)=f(a^nx).$$ ...