Definition:Addition

Let $$\left({S, \circ; \preceq}\right)$$ be a naturally ordered semigroup.

Then the operation $$\circ$$ in $$\left({S, \circ; \preceq}\right)$$ is called addition.

Natural Numbers
The addition operation in the domain of natural numbers $$\mathbb{N}$$ is written $$+$$.

As the Natural Numbers are a Naturally Ordered Semigroup, it is appropriate to write this set as $$\left({\mathbb{N}, +; \le}\right)$$.

From this, it can be seen that $$+$$ corresponds with the operation $$\circ$$ in the structure $$\left({S, \circ; \preceq}\right)$$.

Integers
The addition operation in the domain of integers $$\mathbb{Z}$$ is written $$+$$.

As the set of integers is the Inverse Completion of Natural Numbers, it follows that elements of $$\mathbb{Z}$$ are the isomorphic images of the elements of equivalence classes of $$\mathbb{N} \times \mathbb{N}$$ where two tuples are equivalent if the Unique Minus between the two elements of each tuple is the same.

Thus addition can be formally defined on $$\mathbb{Z}$$ as the operation induced on those equivalence classes as specified in the definition of integers.

That is, the integers being defined as all the Unique Minus congruence classes, integer addition can be defined directly as the operation induced by natural number addition on these congruence classes:

$$\forall \left({a, b}\right), \left({c, d}\right) \in \mathbb{N} \times \mathbb{N}: \left[\!\left[{a, b}\right]\!\right]_\boxminus + \left[\!\left[{c, d}\right]\!\right]_\boxminus = \left[\!\left[{a + c, b + d}\right]\!\right]_\boxminus$$

Integers Modulo m
The addition operation on $$\mathbb{Z}_m$$, the set of integers modulo $m$, is defined by the rule:

$$\left[\!\left[{a}\right]\!\right]_m +_m \left[\!\left[{b}\right]\!\right]_m = \left[\!\left[{a + b}\right]\!\right]_m$$.

Although the operation of addition modulo $$m$$ is denoted by the symbol $$+_m$$, if there is no danger of confusion, the symbol $$+$$ is often used instead.

More usually, though, the notation $$a + b \left({\bmod\, m}\right)$$ is used instead of $$\left[\!\left[{a}\right]\!\right]_m +_m \left[\!\left[{b}\right]\!\right]_m$$.

It means the same thing and, although obscuring the true meaning behind modulo arithmetic, is more streamlined and less unwieldy.

Rational Numbers
The addition operation in the domain of rational numbers $$\mathbb{Q}$$ is written $$+$$.

Let $$a = \frac p q, b = \frac r s$$ where $$p, q \in \mathbb{Z}, r, s \in \mathbb{Z} - \left\{{0}\right\}$$.

Then $$a + b$$ is defined as $$\frac p q + \frac r s = \frac {p s + r q} {q s}$$.

This definition follows from the definition of and proof of existence of the quotient field of any integral domain, of which the set of integers is one.

Real Numbers
The addition operation in the domain of real numbers $$\mathbb{R}$$ is written $$+$$.

Complex Numbers
The addition operation in the domain of complex numbers $$\mathbb{C}$$ is written $$+$$.

Let $$z = a + \imath b, w = c + \imath d$$ where $$a, b, c, d \in \mathbb{R}$$.

Then $$z + w$$ is defined as $$\left({a + \imath b}\right) + \left({c + \imath d}\right) = \left({a + c}\right) + \imath \left({b + d}\right)$$.