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What is a prime number?
How to find prime
numbers
• A prime number can be divided only by itself
and 1 with nothing left over.
• It has exactly two factors. A factor is a whole
number
b which
hi h di
divides
id iinto a target number
b with
ih
no remainder. For example, the factors of 12 are
1, 2, 3, 4, 6, 12 because I can divide 12 by all
of them with no remainder.
Using the sieve of Eratosthenes.
(Invented in Greece 2300 years ago
but still works today!)
• This means that 1 is not a prime number.
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How to use The Sieve of Eratosthenes.
How to use The Sieve of Eratosthenes.
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Begin with a number
grid. It can contain
as many numbers as
you like.
LLet’s start withh a
small one.
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We are going to
cross off all of the
numbers that are
NOT prime numbers.
L
Let’s
start ffrom the
h
smallest number.
Remember, 1 isn’t a
prime number.
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How to use The Sieve of Eratosthenes.
How to use The Sieve of Eratosthenes.
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Now we’ll look at the
next smallest number
which is 2.
We need to circle it
as it is a prime
number.
Next we blot out all
of the numbers which
are multiples of 2.
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Ok, 3 is the next
smallest number now.
We need to circle it.
Now we blot out all
the
h multiples
l l off 3
3.
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How to use The Sieve of Eratosthenes.
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The sieve of Eratosthenes
is simple, yet very clever.
It can be used to work
out any prime numbers,
as long
l
as you hhave
patience.
Try to explain how we
can be sure that it works
– what is going on?
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