Find the sum of the smallest three prime factors of
6 2 2 − 1
For example, if the number was 9 0 , or 2 1 3 2 5 1 , the answer would be 2 + 3 + 5 = 1 0 .
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Use \mid.
Note that you still have to explain why no smaller prime works.
How did u do the factorization x^22-y^22 ?
will 11 be another factor?
Did You use binomial theorem for this factorization?
6²²-1= 5×7×23×3154757×51828151
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Correct! Way of present generation for this particular question.
We are looking for the smallest three primes m where 6 2 2 ≡ 1 ( m o d m ) .
Obviously, 2 and 3 are not valid options, so we next look to 5.
It so happens that 6 ≡ 6 2 2 ≡ 1 ( m o d 5 ) ⟹ 5 ∣ 6 2 2 , and there we have our first prime.
Logically, the next step would be to look at 7. 6 ≡ − 1 ( m o d 7 ) ⟹ 6 2 ≡ 1 ( m o d 7 ) ⟹ ( 6 2 ) 1 1 ≡ 1 ( m o d 7 ) ⟹ 7 ∣ 6 2 2 . So we have our second prime.
Note: More generally, if you have a number a n − 1 , it would always be divisible by a − 1 , and if n is an even number, a + 1 would also be a factor.
If you keep working up the primes (11, 13, etc), you, like me, would realize that you are just wasting your time because it doesn't work out and there is no pattern whatsoever.
So I got bored and turned to Fermat's Little Theorem. 6 2 2 = 6 2 3 − 1 ≡ 1 ( m o d 2 3 ) ⟹ 2 3 ∣ 6 2 2 .
Therefore, the answer is 5 + 7 + 2 3 = 3 5 .
Why you stoped at three numbers ? why not you add 11 ?
Bash the mods of the first primes and find that the first 3 that work are 5,7,and 23. Sum them to get 35.
But what does bash the mods mean?
Correct. Thank you for this solution. The key was to use Fermat's Little Theorem. :P
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I wrote that solution.
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Yes, you did. Thanks! :D
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@Finn Hulse – Also you need to bash all primes under 2 3 to make sure that 2 3 is the third smallest. I didn't like that part, it wasn't very elegant.
Neat trick. I programmed a python prime factorization algorithm to solve.
Works like a charm.
Worst solution ever!
Why to use a program every time?Apply your brains
Checking cases in ascending order gives us 5 and 7 as primefactors. The rest upto 19 can be checked using modular arithematic. for 23, Observe that 22=23-1 i.e. a PRIME! So, Check if fermat's little theorem can be used.
= 131621703842267135 = 5 x 7 x 23 x 163505222164307
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First we shall factor x 2 2 − y 2 2
You get:
( x − y ) ( x + y ) ( x 1 0 − x 9 y + x 8 y 2 − x 7 y 3 + x 6 y 4 − x 5 y 5 + x 4 y 6 − x 3 y 7 + x 2 y 8 − x y 9 + y 1 0 ) ( x 1 0 + x 9 y + x 8 y 2 + x 7 y 3 + x 6 y 4 + x 5 y 5 + x 4 y 6 + x 3 y 7 + x 2 y 8 + x y 9 + y 1 0 )
Now, plugging in the values of x and y with 6 and 1, you get 5 and 7 for the first two factors.
We need another factor:
Consider ( 6 2 2 − 1 ) . Note that 22 = 23 -1 and 23 is prime
By Fermat's Little theorem,
6 2 2 ≡ 6 2 3 − 1 ≡ 1 ( m o d 2 3 )
⟹ 2 3 ∣ ( 6 2 2 − 1 )
So, the other factor is 2 3
TL;DR Go to Wolfram|Alpha and type
which gives
If you liked my solution, please upvote and tell me what is the correct way to write divides in LaTeX