# Prime numbers and prime factor trees

Erastothene’s sieve interactive

Prime numbers are special numbers with exactly two factors. Erastothene’s sieve is a way of finding those prime numbers by removing the multiples of numbers. You can see them above. Prime factor trees are useful to show the numbers that a whole number can be reduced to.

# Prime Numbers and Factors

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Today we are going to use Erastothene’s Sieve to find the prime numbers up to 100. You will use the “Count By” app on the iPods or iPads or your netbook with the interactive 100 chart program. Color in the corresponding numbers on your paper 100 chart.

Prime numbers are numbers with exactly two factors – you can divide a prime number by one or itself, but no other number, to get a whole number. The number one only has one factor, so it is not a prime number. So, 2 and 3 are the first two prime numbers. 4 is not a prime number because it has three factors (1, 2 and 4). Circle the prime numbers on your paper chart and stick it firmly in your work book to refer to later.