Additive and Multiplicative Functions
Given a function $f : X \to Y$, let $a, b \in X$, $a_{i} \in X\ (i=1,\cdots)$.
Subadditive Function
A function $f$ is called a subadditive function when it satisfies the following equation:
$$ f(a+b) \le f(a)+f(b) $$
The absolute value is an example.
$$ |3+(-4)| \le |3|+|-4| $$
Another example, if we have $f(x)=2x+3$ then
$$ 13=f(2+3) \le f(2)+f(3)=7+9=16 $$
Additive Function
A function $f$ is called an additive function when it satisfies the following equation:
$$ f(a+b) = f(a)+f(b) $$
This is the case where equality holds in subadditivity.
For example, if $f(x)=4x$
$$ 20=f(2+3)=f(2)+f(3)=20 $$
If set $E_{1},\ E_2$ satisfies $E_{1} \cap E_2 = \emptyset$ and let the number of elements in $n(E_{i})=E_{i}$ be
$$ n(E_{1} \cup E_2) = n(E_{1}) + n(E_2) $$
Countably Subadditive Function
A function $f$ is called a countably subadditive function when it satisfies the following equation:
$$ f \left( \sum_{i=1}^\infty a_{i} \right) \le \sum \limits_{i=1}^\infty f(a_{i}) $$
From subadditivity and additivity, it can be seen that this also holds for any arbitrary number of $N$ elements. If it holds for a countable number of elements, it is said to have countable subadditivity. An example of countable subadditivity is outer measure.
Countably Additive Function
A function $f$ is called a countably additive function when it satisfies the following equation:
$$ f \left( \sum_{i=1}^\infty a_{i} \right) = \sum \limits_{i=1}^\infty f(a_{i}) $$
This is the case where equality holds in countable subadditivity.
For distinct elements, the outer measure is countably additive. If $E_{i} \cap E_{j} =\emptyset \quad \forall\ i,j$ then
$$ \mu^{\ast} \left( \bigsqcup _{i=1}^\infty E_{i} \right) = \sum _{i=1}^\infty \mu^{\ast}(E_{i}) $$
Submultiplicative Function
A function $f$ is called a submultiplicative function when it satisfies the following equation:
$$ f(ab) \le f(a)f(b) $$
This applies the same properties of addition to multiplication.
Multiplicative Function
A function $f$ is called a multiplicative function when it satisfies the following equation:
$$ f(ab) = f(a)f(b) $$
This is the case where equality holds in submultiplicativity.