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Reply Thu 29 Nov, 2012 11:17 pm
Prove that a prime number cannot be expressed as the sum of two or more consecutive positive odd integers.
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Type: Question • Score: 3 • Views: 789 • Replies: 5
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Ticomaya
 
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Reply Thu 29 Nov, 2012 11:19 pm
@jennyochoa1,
You first.
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laughoutlood
 
  1  
Reply Fri 30 Nov, 2012 01:38 am
@jennyochoa1,
You want that in numbers or letters?

Two odd numbers (or a factor of two odd numbers) adds to an even number therefore it is not prime.

Three odd numbers (or more than 3 odd numbers) requires than 1 of the odd numbers is more than one either side of a number divisible by six, therefore at best two are either side of a six and the third is 3 away from a six therefore it is divisible by 3.

qed
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raprap
 
  1  
Reply Fri 30 Nov, 2012 03:44 am
@jennyochoa1,
1) The sum of two consecutive odd integers is even.
let n be any even number
then (n+1) and (n+3) are two consecutive odd numbers.
(n+1)+(n+3)=2n+4=2(n+2)

2) The only even prime numer is two (2).

3) Two (2) cannot be expresses as the sum of two consecutive odd integers.
the smallest even/odd number is 0, so let n=0, then (n+1)=1 and (n+3)=3
1+3=4
4 is not equal to 2

4) The sum of two consecutive odd integers is not 2

5) The sum of any two consecutive odd integers cannot be prime.

Rap



laughoutlood
 
  1  
Reply Fri 30 Nov, 2012 10:43 pm
@raprap,
wow rap you've really clobbered two odd numbers any chance of you addressing 3 or more odd numbers, that is the question?

i reckon my answer was as elegant as a mathematician on a bicycle
raprap
 
  1  
Reply Sat 1 Dec, 2012 05:55 am
@laughoutlood,
proving the sum of more than one consecutive odd numbers is not prime is somewhat trivial.

simply show that the sum of k consecutive odd itegers is

(n+1)+(n+3)+......+(n+1+(k-1)*2)=k*n+(1+3+...+(1+(k-1)*2)=k*n+k^2=k(n+k)

BTW one of my cycling buds has a phd in math---

Rap
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