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# Prime Number Test - Proof please?

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Manager
Joined: 22 Feb 2012
Posts: 93

Kudos [?]: 25 [0], given: 25

Schools: HBS '16
GMAT 1: 740 Q49 V42
GMAT 2: 670 Q42 V40
GPA: 3.47
WE: Corporate Finance (Aerospace and Defense)

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19 Mar 2012, 09:46
Found this in a Advanced GMAT Quant book....

Havent seen this anywhere else and would be interested in the proof for it if anyone can supply...

For all Prime numbers, p > 5

p^2 -1 is divisible by 24

Kudos [?]: 25 [0], given: 25

Current Student
Joined: 12 Sep 2011
Posts: 900

Kudos [?]: 958 [1], given: 114

Concentration: Finance, Finance
GMAT 1: 710 Q48 V40

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19 Mar 2012, 10:28
1
KUDOS
I have also never seen this formula before, but as we all know, prime numbers have some very creative tricks and rules associated with them. Since the number resulting from this formula will always be a factor of 24 then that means that the answer to (P^2)-1 will always have 3 factors of 2 and 1 factor of 3 (aka 3 * 2^3).

Prime numbers will always be odd, and a prime number ^2 will also always be odd. So to subtract one from that number will always give you an even number. Since the prime number has be to greater than 5, then the lowest number this formula will yield is 48. Any even number great than 4 that does not have 2 and a prime number as its only factor will always have at least 3 factors of 2. So the last number that has to be dealt with to explain this formula is the last factor of 3. As you can see below, every number is divisible by 3 which explains this. Unfortunately, I can not figure out why this formula will always yield a number divisible by 3. Does anyone else have any insight? Very interesting one.

7^2 = 49 - 1 = 48
11^2 = 121 - 1 = 120
13^2 = 169 - 1 = 168
17^2 = 289 - 1 = 288
19^2 = 361 - 1 = 360
23^2 = 529 - 1 = 528
29^2 = 841 - 1 = 840

To me, this formula is just one of the "it is what it is" type formulas. The proof could be very complicated, and in fact be no real help to you on test day. My advice is to commit this formula to memory, and if you see anything to do with this on test day, you will be golden. Most likely you will not see anything referring to this formula, but the more tricks, tips, patterns, and secrets like these you know, the more time you can save and the higher score you can achieve.
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CEO
Joined: 17 Nov 2007
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Schools: Chicago (Booth) - Class of 2011
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19 Mar 2012, 10:33
3
KUDOS
Expert's post
S = p^2-1 = (p-1)*(p+1)

1) if p is a prime number and p>5, p is an odd number and both p-1 and p+1 are even integers and one of them is divisible by 4. So, S is divisible by 2*4=8

2) if we have 3 consecutive integers: (p-1), p, (p+1), one of them must be divisible by 3. p can't be divisible by 3 as it's a prime integer and greater than 5. Therefore, one of (p-1) or (p+1) is divisible by 3 and S is divisible by 3.

3) from 1&2 S is divisible by 3*8=24.
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Kudos [?]: 4586 [3], given: 360

Current Student
Joined: 12 Sep 2011
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Kudos [?]: 958 [0], given: 114

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GMAT 1: 710 Q48 V40

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19 Mar 2012, 10:40
Well done walker! You are correct... I guess this is why I got a measly Q48

Great insight

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19 Mar 2012, 10:40
AbeinOhio wrote:
Found this in a Advanced GMAT Quant book....

Havent seen this anywhere else and would be interested in the proof for it if anyone can supply...

For all Prime numbers, p > 5

p^2 -1 is divisible by 24

A prime number greater than 3 is always of the form (6n+1) or (6n - 1) where n is a positive integer. Mind you, every number of the form (6n+1) or (6n - 1) is not prime but every prime (greater than 3) is of one of these forms. e.g. 5 = 6*1 - 1; 7 = 6*1+1; 11 = 6*2 - 1 etc

p^2 - 1 = (p-1)(p+1)

Since p is odd, (p is prime greater than 5), both (p-1) and (p+1) are even. Since (p-1) and (p+1) are consecutive even numbers, one of them will be divisible by 4. So (p-1)(p+1) has 8 as a factor.
Since p is of the form (6n+1) or (6n - 1), one of (p-1) and (p+1) must be of the form 6n i.e. it must be divisible by 6.
Hence, (p-1)(p+1) has 3 as a factor.

Therefore, (p-1)(p+1) has 8*3 = 24 as a factor.

These are not important rules/formulas. You can deduce these from other things you know but then there are unlimited properties you can deduce. So just focus on learning the basics.
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19 Mar 2012, 19:40
Walker respect man!!!!1

Have one question though can negative number be prime? like -5, -3?

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Schools: Chicago (Booth) - Class of 2011
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19 Mar 2012, 19:56
No.

A prime number (or a prime) is a natural number greater than 1 that has no positive divisors other than 1 and itself.
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Re: Prime Number Test - Proof please?   [#permalink] 19 Mar 2012, 19:56
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