Equation whose unknown is a function
In
mathematics , a functional equation
[1]
[2] [
irrelevant citation ] is, in the broadest meaning, an
equation in which one or several functions appear as
unknowns . So,
differential equations and
integral equations are functional equations. However, a more restricted meaning is often used, where a functional equation is an equation that relates several values of the same function. For example, the
logarithm functions are
essentially characterized by the logarithmic functional equation
log
(
x
y
)
=
log
(
x
)
+
log
(
y
)
.
{\displaystyle \log(xy)=\log(x)+\log(y).}
If the
domain of the unknown function is supposed to be the
natural numbers , the function is generally viewed as a
sequence , and, in this case, a functional equation (in the narrower meaning) is called a
recurrence relation . Thus the term functional equation is used mainly for
real functions and
complex functions . Moreover a
smoothness condition is often assumed for the solutions, since without such a condition, most functional equations have very irregular solutions. For example, the
gamma function is a function that satisfies the functional equation
f
(
x
+
1
)
=
x
f
(
x
)
{\displaystyle f(x+1)=xf(x)}
and the initial value
f
(
1
)
=
1.
{\displaystyle f(1)=1.}
There are many functions that satisfy these conditions, but the gamma function is the unique one that is
meromorphic in the whole complex plane, and
logarithmically convex for x real and positive (
Bohr–Mollerup theorem ).
Examples
Recurrence relations can be seen as functional equations in functions over the integers or natural numbers, in which the differences between terms' indexes can be seen as an application of the
shift operator . For example, the recurrence relation defining the
Fibonacci numbers ,
F
n
=
F
n
−
1
+
F
n
−
2
{\displaystyle F_{n}=F_{n-1}+F_{n-2}}
, where
F
0
=
0
{\displaystyle F_{0}=0}
and
F
1
=
1
{\displaystyle F_{1}=1}
f
(
x
+
P
)
=
f
(
x
)
{\displaystyle f(x+P)=f(x)}
, which characterizes the
periodic functions
f
(
x
)
=
f
(
−
x
)
{\displaystyle f(x)=f(-x)}
, which characterizes the
even functions , and likewise
f
(
x
)
=
−
f
(
−
x
)
{\displaystyle f(x)=-f(-x)}
, which characterizes the
odd functions
f
(
f
(
x
)
)
=
g
(
x
)
{\displaystyle f(f(x))=g(x)}
, which characterizes the
functional square roots of the function g
f
(
x
+
y
)
=
f
(
x
)
+
f
(
y
)
{\displaystyle f(x+y)=f(x)+f(y)\,\!}
(
Cauchy's functional equation ), satisfied by
linear maps . The equation may, contingent on the
axiom of choice , also have other pathological nonlinear solutions, whose existence can be proven with a
Hamel basis for the real numbers
f
(
x
+
y
)
=
f
(
x
)
f
(
y
)
,
{\displaystyle f(x+y)=f(x)f(y),\,\!}
satisfied by all
exponential functions . Like Cauchy's additive functional equation, this too may have pathological, discontinuous solutions
f
(
x
y
)
=
f
(
x
)
+
f
(
y
)
{\displaystyle f(xy)=f(x)+f(y)\,\!}
, satisfied by all
logarithmic functions and, over coprime integer arguments,
additive functions
f
(
x
y
)
=
f
(
x
)
f
(
y
)
{\displaystyle f(xy)=f(x)f(y)\,\!}
, satisfied by all
power functions and, over coprime integer arguments,
multiplicative functions
f
(
x
+
y
)
+
f
(
x
−
y
)
=
2
f
(
x
)
+
f
(
y
)
{\displaystyle f(x+y)+f(x-y)=2[f(x)+f(y)]\,\!}
(quadratic equation or
parallelogram law )
f
(
(
x
+
y
)
/
2
)
=
(
f
(
x
)
+
f
(
y
)
)
/
2
{\displaystyle f((x+y)/2)=(f(x)+f(y))/2\,\!}
(
Jensen's functional equation )
g
(
x
+
y
)
+
g
(
x
−
y
)
=
2
g
(
x
)
g
(
y
)
{\displaystyle g(x+y)+g(x-y)=2[g(x)g(y)]\,\!}
(
d'Alembert's functional equation )
f
(
h
(
x
)
)
=
h
(
x
+
1
)
{\displaystyle f(h(x))=h(x+1)\,\!}
(
Abel equation )
f
(
h
(
x
)
)
=
c
f
(
x
)
{\displaystyle f(h(x))=cf(x)\,\!}
(
Schröder's equation ).
f
(
h
(
x
)
)
=
(
f
(
x
)
)
c
{\displaystyle f(h(x))=(f(x))^{c}\,\!}
(
Böttcher's equation ).
f
(
h
(
x
)
)
=
h
′
(
x
)
f
(
x
)
{\displaystyle f(h(x))=h'(x)f(x)\,\!}
(
Julia's equation ).
f
(
x
y
)
=
∑
g
l
(
x
)
h
l
(
y
)
{\displaystyle f(xy)=\sum g_{l}(x)h_{l}(y)\,\!}
(Levi-Civita),
f
(
x
+
y
)
=
f
(
x
)
g
(
y
)
+
f
(
y
)
g
(
x
)
{\displaystyle f(x+y)=f(x)g(y)+f(y)g(x)\,\!}
(
sine addition formula and
hyperbolic sine addition formula ),
g
(
x
+
y
)
=
g
(
x
)
g
(
y
)
−
f
(
y
)
f
(
x
)
{\displaystyle g(x+y)=g(x)g(y)-f(y)f(x)\,\!}
(
cosine addition formula ),
g
(
x
+
y
)
=
g
(
x
)
g
(
y
)
+
f
(
y
)
f
(
x
)
{\displaystyle g(x+y)=g(x)g(y)+f(y)f(x)\,\!}
(
hyperbolic cosine addition formula ).
The
commutative and
associative laws are functional equations. In its familiar form, the associative law is expressed by writing the
binary operation in
infix notation ,
(
a
∘
b
)
∘
c
=
a
∘
(
b
∘
c
)
,
{\displaystyle (a\circ b)\circ c=a\circ (b\circ c)~,}
but if we write f (a , b ) instead of a ○ b then the associative law looks more like a conventional functional equation,
f
(
f
(
a
,
b
)
,
c
)
=
f
(
a
,
f
(
b
,
c
)
)
.
{\displaystyle f(f(a,b),c)=f(a,f(b,c)).\,\!}
The functional equation
f
(
s
)
=
2
s
π
s
−
1
sin
(
π
s
2
)
Γ
(
1
−
s
)
f
(
1
−
s
)
{\displaystyle f(s)=2^{s}\pi ^{s-1}\sin \left({\frac {\pi s}{2}}\right)\Gamma (1-s)f(1-s)}
is satisfied by the
Riemann zeta function , as proved
here . The capital Γ denotes the
gamma function .
The gamma function is the unique solution of the following system of three equations:[
citation needed ]
f
(
x
)
=
f
(
x
+
1
)
x
{\displaystyle f(x)={f(x+1) \over x}}
f
(
y
)
f
(
y
+
1
2
)
=
π
2
2
y
−
1
f
(
2
y
)
{\displaystyle f(y)f\left(y+{\frac {1}{2}}\right)={\frac {\sqrt {\pi }}{2^{2y-1}}}f(2y)}
f
(
z
)
f
(
1
−
z
)
=
π
sin
(
π
z
)
{\displaystyle f(z)f(1-z)={\pi \over \sin(\pi z)}}
(
Euler's
reflection formula )
The functional equation
f
(
a
z
+
b
c
z
+
d
)
=
(
c
z
+
d
)
k
f
(
z
)
{\displaystyle f\left({az+b \over cz+d}\right)=(cz+d)^{k}f(z)}
where a , b , c , d are
integers satisfying
a
d
−
b
c
=
1
{\displaystyle ad-bc=1}
, i.e.
|
a
b
c
d
|
{\displaystyle {\begin{vmatrix}a&b\\c&d\end{vmatrix}}}
= 1, defines f to be a
modular form of order k .
One feature that all of the examples listed above[
clarification needed ] share in common is that, in each case, two or more known functions (sometimes multiplication by a constant, sometimes addition of two variables, sometimes the
identity function ) are inside the argument of the unknown functions to be solved for.[
citation needed ]
When it comes to asking for all solutions, it may be the case that conditions from
mathematical analysis should be applied; for example, in the case of the Cauchy equation mentioned above, the solutions that are
continuous functions are the 'reasonable' ones, while other solutions that are not likely to have practical application can be constructed (by using a
Hamel basis for the
real numbers as
vector space over the
rational numbers ). The
Bohr–Mollerup theorem is another well-known example.
Involutions
The
involutions are characterized by the functional equation
f
(
f
(
x
)
)
=
x
{\displaystyle f(f(x))=x}
. These appear in
Babbage's functional equation (1820),
[3]
f
(
f
(
x
)
)
=
1
−
(
1
−
x
)
=
x
.
{\displaystyle f(f(x))=1-(1-x)=x\,.}
Other involutions, and solutions of the equation, include
f
(
x
)
=
a
−
x
,
{\displaystyle f(x)=a-x\,,}
f
(
x
)
=
a
x
,
{\displaystyle f(x)={\frac {a}{x}}\,,}
and
f
(
x
)
=
b
−
x
1
+
c
x
,
{\displaystyle f(x)={\frac {b-x}{1+cx}}~,}
which includes the previous three as special cases or limits.
Solution
One method of solving elementary functional equations is substitution.[
citation needed ]
Some solutions to functional equations have exploited
surjectivity ,
injectivity ,
oddness , and
evenness .[
citation needed ]
Some functional equations have been solved with the use of
ansatzes ,
mathematical induction .[
citation needed ]
Some classes of functional equations can be solved by computer-assisted techniques.[
vague ]
[4]
In
dynamic programming a variety of successive approximation methods
[5]
[6] are used to solve
Bellman's functional equation , including methods based on
fixed point iterations .
See also
Notes
^ Rassias, Themistocles M. (2000).
Functional Equations and Inequalities . 3300 AA Dordrecht, The Netherlands:
Kluwer Academic Publishers . p. 335.
ISBN
0-7923-6484-8 . {{
cite book }}
: CS1 maint: location (
link )
^ Czerwik, Stephan (2002).
Functional Equations and Inequalities in Several Variables . P O Box 128, Farrer Road, Singapore 912805:
World Scientific Publishing Co. p.
410 .
ISBN
981-02-4837-7 . {{
cite book }}
: CS1 maint: location (
link )
^
Ritt, J. F. (1916). "On Certain Real Solutions of Babbage's Functional Equation". The Annals of Mathematics . 17 (3): 113–122.
doi :
10.2307/2007270 .
JSTOR
2007270 .
^ Házy, Attila (2004-03-01). "Solving linear two variable functional equations with computer". Aequationes Mathematicae . 67 (1): 47–62.
doi :
10.1007/s00010-003-2703-9 .
ISSN
1420-8903 .
S2CID
118563768 .
^ Bellman, R. (1957). Dynamic Programming,
Princeton University Press .
^ Sniedovich, M. (2010). Dynamic Programming: Foundations and Principles,
Taylor & Francis .
References
János Aczél ,
Lectures on Functional Equations and Their Applications ,
Academic Press , 1966, reprinted by Dover Publications,
ISBN
0486445232 .
János Aczél & J. Dhombres,
Functional Equations in Several Variables ,
Cambridge University Press , 1989.
C. Efthimiou, Introduction to Functional Equations , AMS, 2011,
ISBN
978-0-8218-5314-6 ;
online .
Pl. Kannappan,
Functional Equations and Inequalities with Applications , Springer, 2009.
Marek Kuczma ,
Introduction to the Theory of Functional Equations and Inequalities , second edition, Birkhäuser, 2009.
Henrik Stetkær,
Functional Equations on Groups , first edition, World Scientific Publishing, 2013.
Christopher G. Small (3 April 2007).
Functional Equations and How to Solve Them . Springer Science & Business Media.
ISBN
978-0-387-48901-8 .
External links