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Prime Numbers and
Factors
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An integer (greater than one) is prime if the only whole numbers it
can be divided by (without a remainder) are itself and one. All
other integers are composite. In other words, a prime number has
only two positive factors. Composite numbers have more. For example,
seven is a prime number because its only positive factors are one
and seven. Fifteen is composite because it has four: one, three,
five, and fifteen.
Eratosthenes' Prime Number Sieve
Eratosthenes was a Greek mathematician who figured
out that to find all the prime numbers between two and some large
number, you need to remove all the multiples of each number between
two and your large number.
Dr. Math: Prime Numbers
"A prime number is a positive integer that has
exactly two positive integer factors, 1 and itself. For example, if
we list the factors of 28, we have 1, 2, 4, 7, 14, and 28. That's
six factors. If we list the factors of 29, we only have 1 and 29.
That's 2. So we say that 29 is a prime number, but 28 isn't." Dr.
Math presents an excellent introduction to prime numbers, the Sieve
of Eratosthenes, and links to other prime number sites.
Fact Monster: Prime Numbers
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