Download Eratosthenes - who was he, why a sieve and what`s a prime number?

Eratosthenes’ Sieve
This is a method to discover prime numbers.
What is a prime number??
A multiple of a number can be divided exactly by that number.
e.g. 6,21 and 30 are all multiples of 3.
A factor of a number is something that divides exactly into it.
e.g. 2,3,4 and 6 are all factors of 12.
A prime number has only two factors – itself and 1.
e.g. 2, 5 and 7 are all prime numbers.*
1 is not a prime number as it only has one factor – 1!
Who was Eratosthenes?
Eratosthenes (276 B.C. - 196 B.C.) Eratosthenes
was born in Cyrene, now Libya, in North Africa. He was
a Greek mathematician, astronomer, geographer and
poet, who is credited with measuring the circumference
of the earth
His sieve drains out all numbers except for prime
numbers.
How to use the Sieve
1. Draw a grid numbered 1 -100 (there is one below that you can print out).
2. Cross out 1 as it is not a prime number.
3. Put a circle round 2 and cross out all multiples of 2, for example 4, 12, 22 –
in fact all even numbers.
4. Put a circle round 3 and cross out all multiples of 3, for example 6, 9, 18.
5. Put a circle round 5 and cross out all multiples of, for example 15, 35. You
should find that all the multiples that end in 0 have already gone, thanks to 2.
6. Now you should have the idea. Carry on with the next number and so on
until you have finished. You will find there are plenty of larger prime numbers
but that any multiples are bigger than 100. I have given you some helpful
tables at the bottom.
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99 100
Helpful
tables
1
2
3
4
5
6
7
13
26
39
52
65
78
91
17
34
51
68
85
19
38
57
76
95
23
46
69
92
29
58
87
Now you can try a bigger square if you like, for example, up to 400.
* Extract from : Haighton, J., Holder, D., Phillips, B., Thomas, V. (2004) p.36, Maths The Basic Skills
Cheltenham; Nelson Thornes Ltd