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What criteria make a number a prime number?

What is a prime number? How do I recognize a prime number? what makes 7, for instance, a prime number?

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It is any number only divisible by itself and 1 (giving whole number results). A list here of the first 1000

Posted on May 07, 2014


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Is 91 a prime number

No, 91 is not a prime number, it is a composite number.
Why 91 is not a prime number...

It is Because 91 has divisors rather than 1 and itself. 91 can be divided by 7 and 13.

Jul 19, 2017 | Prime Computers & Internet

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Is 8 a prime number? And what are the factors for 20? And is 20 a prime number?

No it isn't as 1,2,4,8 can be used to divide it prime's are only dividable by 1 or itself only
20 is 1,2,4,5,10

Jan 23, 2017 | The Computers & Internet

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Here we go
52 - can be divided by 2 - therefore NOT Prime
63 - can be divided by 3 - therefore NOT Prime
75 - can be divided by 5 - therefore NOT Prime
31 - cannot be divided by whole numbers - therefore Prime
77 - can be divided by 7 - therefore NOT Prime
59 - cannot be divided by whole numbers - therefore Prime
87 - cannot be divided by whole numbers - therefore Prime
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Mar 08, 2015 | Computers & Internet

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How do you identify prime numbers

One trick I use is to add up all the digits and divide by 3. If the answer is even, (a multiple of 3), it's not a prime number.
57 = 5+7=12. 12 divides by 3 evenly (4), is not a prime.
59 = 5+9=14. 14 does not divide by 3 evenly, therefore prime.
999 = 9+9+9=27 divide by 3 = 9, not prime
997 = 9+9+7=25 divide by 3=8.3 not even and is prime

May 30, 2014 | Computers & Internet

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My casio fx-83GT Plus calculator wont do HCF for me

There is nothing you can do to make the calculator find the HCF for you. No point complaining about that. But if you are interested in doing it by hand (using the calculator to do the divisions for you) here how it is done.
  1. Decompose the first number in prime factors. If a prime factor is repeated use the exponent notation: That helps.
  2. Decompose the second number in prime factors too, using the exponent notation.
  3. Now look at the two decompositions. If a prime factor is present in both decompositions it must be in the HCD /HCF, with the smallest of its two exponents.
  4. Do that for all prime factors
Example: Here are the decompositions of two numbers
(2^5)*3*(5^4)(7^3)*11 and (2^3)*(5^6)*(11^2)*7
The prime factors that are present in both decompositions are
2, 5, 7, and 11
From the two decompositions select the smallest power of each common prime factor. They are represented in bold fonts.
2^3, 5^4, 7, 11
The highest common divisor/Highest common factor is

Mar 27, 2014 | Casio Office Equipment & Supplies

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I cant understand prime numbers

A prime number is a positive whole number (natural number) which cannot be divided into smaller whole numbers. In short, it can only be divided (integer division) by itself and 1.
First prime number is 2, then 3, 5, 7, 11, 13, 17, 19,
To generate a list of prime numbers the Greek Eratoshenes used a method called now the sieve Eratosthenes.

  1. Write all the natural numbers up to some chosen limit (100, 259, any limit )
  2. Remove 1 by crossing it ( 1 is no longer considered prime).
  3. The first prime number is 2. Circle the number 2.
  4. Go through the whole list crossing out all the numbers that are multiple of 2, that is 4, 6, 8,10,12, ...
  5. Repeat.
  6. The next prime number is 3 since it was not crossed out as a multiple of 2. Circle 3.
  7. Go through the whole list crossing all the numbers that are multiples of 3, and which have not already been crossed out as multiples of 2 .
  8. Number 4 has been crossed out already.
  9. Next prime number is 5. Circle 5
  10. Go through the whole list crossing all the numbers that are multiple of 5 and which have not been crossed out already as multiples of 2, or 3
  11. Repeat:
  12. Next prime number is 7. Circle 7
  13. Go through the whole list again, crossing out the multiples of 7, and so on.
If you want to test the primality of a positive integer (odd number) starting by dividing it by 3 (use divisibility rules or otherwise), then divide by 5, by seven, and all subsequent prime numbers. If it is not divisible by any prime number, keep going but do not exceed the prime number that is closest yet smaller than square root of your number.


Nov 26, 2013 | Office Equipment & Supplies

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3 is a prime number because it cannot be divided evenly by any whole number other than 1 and itself. A prime number is any whole number which cannot be divided evenly by any whole number other than one and itself.

Oct 02, 2013 | Panini Maurice Jones-drew 2011 Prestige...

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Whats prime numbers

Prime numbers are those numbers (greater than 1) that cannot be divided by any number except themselves and one.
The Greek Eratosthenes created a method to find out these prime numbers, although it only worked over a limited range:
1) Write out the numbers from 1 to 100 in ten rows of 10.

hundsq.gif2) Cross off number 1, because all primes are greater than 1.
3) Number 2 is a prime, so we can keep it, but we need to cross off the multiples of 2 (i.e. even numbers).
4) Number 3 is also a prime, so again we keep it and cross off the multiples of 3.
5) The next number left is 5 (because four has been crossed off), so we keep it and cross of the multiples of this number.
6) The final number left in the first row is number 7, so cross off its multiples.
7) You have finished. All of the "surviving" numbers (coloured in white below) on your grid are prime numbers.

Oct 23, 2011 | MathRescue Word Problems Of Algebra Lite

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You may want COUNTIF if you're specifying criteria. For instance, if my prices are found in b3 to b7, here's a formula that will find all those that are less than 6 ($6.00):

=COUNTIF(B3:B7, "<6")

If you're using multiple criteria, such as you want to find all the prices that are greater than $5 and less than $8, the following will accomplish it. (The ABS gives you the absolute value of the result, in case the smaller number is first.)

=ABS(SUM(COUNTIF(B3:B7, ">5") - COUNTIF(B3:B7, "<8")))

Nov 06, 2007 | Oracle 10g Database Standard (ODBSEONUPP0)

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