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Each whole number N greater than 1 can be written

as the product of prime numbers . If N is already prime,

the equation just reads N=N. Otherwise, the way of writing N

as a product of primes is unique provided that the primes are

listed in increasing order. Here are the first 20 examples.

2 = 2

3 = 3

4 = 2 times 2

5 = 5

6 = 2 times 3

7 = 7

8 = 2 times 2 times 2

9 = 3 times 3

10 = 2 times 5

11 = 11

12 = 2 times 2 times 3

13 = 13

14 = 2 times 7

15 = 3 times 5

16 = 2 times 2 times 2 times 2

17 = 17

18 = 2 times 3 times 3

19 = 19

20 = 2 times 2 times 5