The operation of **multiplication** is equivalent to repeated addition of the same number.
Thus if a+a+ ... +a = m, where there are b occurrences of a then we say that m is the result of multiplying a by b.
We can **multiply** any two numbers a and b to form a number called their **product**
which we write using a dot on the line a.b read as 'a **times** b'.
(Other notations such as a·b a×b or a*b, or simply juxtaposition, are also used
for multiplication. The asterisk is used in most programming languages.)

Multiplication table (base ten)

. | 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 |

0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 |

1 | 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 |

2 | 0 | 2 | 4 | 6 | 8 | 10 | 12 | 14 | 16 | 18 | 20 |

3 | 0 | 3 | 6 | 9 | 12 | 15 | 18 | 21 | 24 | 27 | 30 |

4 | 0 | 4 | 8 | 12 | 16 | 20 | 24 | 28 | 32 | 36 | 40 |

5 | 0 | 5 | 10 | 15 | 20 | 25 | 30 | 35 | 40 | 45 | 50 |

6 | 0 | 6 | 12 | 18 | 24 | 30 | 36 | 42 | 48 | 54 | 60 |

7 | 0 | 7 | 14 | 21 | 28 | 35 | 42 | 49 | 56 | 63 | 70 |

8 | 0 | 8 | 16 | 24 | 32 | 40 | 48 | 56 | 64 | 72 | 80 |

9 | 0 | 9 | 18 | 27 | 36 | 45 | 54 | 63 | 72 | 81 | 90 |

10 | 0 | 10 | 20 | 30 | 40 | 50 | 60 | 70 | 80 | 90 | 100 |

Like addition, multiplication is commutative: a.b = b.a, and associative: a.(b.c) = (a.b).c. The product of all the numbers in a set or sequence S of numbers is denoted by Π(S). The product of a null set or sequence is conventionally taken to be 1.

Theorem (*Principle of Multiplication of Choices*): __The number of ways of making choices in sequence,
the first from u elements, the second from v elements, the third from w, and so on, is u.v.w....__
Proof: This is little more than the definition of multiplication differently expressed.

Example. The number of choices for the first move by either player in a game of chess is 20 (4 knight moves and 16 pawn moves). Thus the number of ways of playing the first pair of moves is 20.20 = 400.

Theorem (*Selection Principle*): __The number of ways of making a selection from
u things alike of one kind, v of a second kind, w of a third kind, etc, is (u+1).(v+1).(w+1)....__
Proof: From the set of u things alike we can choose u, u–1, u–2, ..., 3, 2, 1 or 0 a total of u+1 cases.
We then apply the previous principle. This count includes the null selection where we choose none from any of the sets.
If this is excluded we must deduct 1 from the total.

The number 1 has the multiplicative identity property: x.1 = x for any x. In particular 1.1 = 1. The number 0 is the multiplicative absorber or nullity: 0.n = 0. In particular 0.0 = 0. If r+n > r.n then either n = 1 or n = 0 with r ≠ 0.

Multiplication is distributive over addition: a.(b+c) = (a.b)+(a.c). This says in effect that ‘a.’ is a linear operator. It is also distributive over subtraction: a.(bHc) = (a.b)H(a.c).

For any numbers n and m (m not 0) there exist unique numbers q and r such that n = q.m + r where r<m.
We call q the **quotient** of n by m, written n|m, and r the **remainder**, written n¬m. We thus have n = (n|m).m + (n¬r).
In the exceptional case when m = 0 we get the relation: n = Q.0 + n where Q can be __any__ number.

Quotient table (base ten)

| | 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 |

0 | Q | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 |

1 | Q | 1 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 |

2 | Q | 2 | 1 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 |

3 | Q | 3 | 1 | 1 | 0 | 0 | 0 | 0 | 0 | 0 | 0 |

4 | Q | 4 | 2 | 1 | 1 | 0 | 0 | 0 | 0 | 0 | 0 |

5 | Q | 5 | 2 | 1 | 1 | 1 | 0 | 0 | 0 | 0 | 0 |

6 | Q | 6 | 3 | 2 | 1 | 1 | 1 | 0 | 0 | 0 | 0 |

7 | Q | 7 | 3 | 2 | 1 | 1 | 1 | 1 | 0 | 0 | 0 |

8 | Q | 8 | 4 | 2 | 2 | 1 | 1 | 1 | 1 | 0 | 0 |

9 | Q | 9 | 4 | 3 | 2 | 1 | 1 | 1 | 1 | 1 | 0 |

10 | Q | 10 | 5 | 3 | 2 | 2 | 1 | 1 | 1 | 1 | 1 |

It will be seen from the table that the values of n|m occur in the table in the form of wedges bounded by straight lines emanating from the top left corner, the upper line of 0s being horizontal, i.e. formed of moves with coordinates (1, 0), the line of 1s diagonally, i.e with move coordinate (1,1), the line of 2s with move coordinates (1,2), like a chess knight, the line of 3s of (1,3) moves, the line of 4s of (1,4) moves and so on.

The remainder r = n¬m is the first number less than m, found by starting with n
and repeatedly subtracting m. This process, the **division algorithm**, gives a
series of numbers: n, n – m, n – (2.m), ..., n – (q.m) each smaller than
the preceding and thus the process terminates when a number is reached that is less
than m. This algorithm provides a way of calculating q and r.

Remainder table (base ten)

¬ | 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 |

0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 |

1 | 1 | 0 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 |

2 | 2 | 0 | 0 | 2 | 2 | 2 | 2 | 2 | 2 | 2 | 2 |

3 | 3 | 0 | 1 | 0 | 3 | 3 | 3 | 3 | 3 | 3 | 3 |

4 | 4 | 0 | 0 | 1 | 0 | 4 | 4 | 4 | 4 | 4 | 4 |

5 | 5 | 0 | 1 | 2 | 1 | 0 | 5 | 5 | 5 | 5 | 5 |

6 | 6 | 0 | 0 | 0 | 2 | 1 | 0 | 6 | 6 | 6 | 6 |

7 | 7 | 0 | 1 | 1 | 3 | 2 | 1 | 0 | 7 | 7 | 7 |

8 | 8 | 0 | 0 | 2 | 0 | 3 | 2 | 1 | 0 | 8 | 8 |

9 | 9 | 0 | 1 | 0 | 1 | 4 | 3 | 2 | 1 | 0 | 9 |

10 | 10 | 0 | 0 | 1 | 2 | 0 | 4 | 3 | 2 | 1 | 0 |

We have the general rules:

n|n = 1, n¬n = 0

n|0 = Q, n¬0 = n

n|1 = n, n¬1 = 0

When n<m then n = 0.m + n so: n|m = 0 and n¬m = n. In the tables these
results correspond to the figures above the main diagonal.

Some rules for manipulation of the remainder operation are:

(n¬m)¬m = n¬m

(k.a)¬a = 0

(n + k.a)¬a = n¬a

(m+n)¬a = (m¬a + n¬a)¬a

(m.n)¬a = (m¬a . n¬a)¬a

**Congruences**: The relation of equality of remainders, a¬m = b¬m, can be
expressed in a number of alternative ways. We can say that a is *congruent*
to b *modulo* m, written a≡b (mod m). Alternatively, since a = p.m + r
and b = q.m + r we can say that a and b are of the same m-*form*, or that
they belong to the same *residue class* mod m. Congruence is reflexive,
symmetric and transitive, i.e. an equivalence relation.