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March 5, 2015

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Posted by **charlie** on Friday, July 30, 2010 at 11:17am.

1 + 3 + 5 + 3 + 1 = 3^2 + 2^2

1 + 3 + 5 + 7 + 9 + 7 + 5+ 3 + 1 = 5^2 + 4^2

1 + 3 + 1 = 2^2 + 1^2

I'm thinking sum of consecutive primes can be expressed in squares? The numbers add up. What's the generality?

Thanks

- Math -
**Reiny**, Friday, July 30, 2010 at 11:44amFirst of all, did you know that the sum of the first n odd numbers is n^2 ?

e.g.

1+3+5+7+9+11 = 6^2 = 36

So when you look at your sequences, they are actually made up of the addition of the sum of n odd number plus the addition of (n-1) odd numbers

notice your second is actually

(1+3+5+7+9) + (1+3+5+7) = 5^2 + 4^2

If you know how to express sigma notation you could do

sigma(2i-1) from i=1 to n + sigma(2i-1) from 1 to n-1 = n^2 - (n-1)^2

- correction - maths -
**Reiny**, Friday, July 30, 2010 at 11:50amlast line should have said :

sigma(2i-1) from i=1 to n + sigma(2i-1) from 1 to n-1 = n^2 + (n-1)^2

- Math -
**charlie**, Friday, July 30, 2010 at 12:00pmThanks Reiny,

Yes, I spotted that it dosen't work for any run of primes, but it does up to 9. I'm afraid i don't really follow your explanation!

An answer for dummies maybe?

Charlie

- Math -
**Reiny**, Friday, July 30, 2010 at 12:40pmok, look at this patters

sum of 1 odd number = 1 = 1^2

sum of 2 odd numbers = 1+3 = 4 = 2^2

sum of 3 odd numbers = 1+3+5 = 9 = 3^2

sum of 4 odd numbers = 1+3+5+7 = 16 = 4^2

...

sum of n odd numbers = 1+3+...+ (2n-1) = n^2

( 2n-1) is the general expression for an odd number.

this has nothing to do with primes.

Why do you say it works only up to 9, it works for the sum of odd numbers up to any value you want.

e.g.

1+3+5+7+9+11+13+15+17+19 = 10^2 = 100.

They were the first 10 odd numbers.

now look again at your second example of

1+3+5+7+9+7+5+3+1

=1+3+5+7+9 + (1+3+5+7)

= sum of first 5 odd numbers + sum of the first 4 odd numbers

= 5^2 + 4^2

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