Actually 3 Problems in 1

Find the sum of the smallest three prime factors of

6 22 1 6^{22}-1

For example, if the number was 90 90 , or 2 1 3 2 5 1 2^13^25^1 , the answer would be 2 + 3 + 5 = 10 2+3+5=10 .


The answer is 35.

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6 solutions

First we shall factor x 22 y 22 x^{22} - y^{22}

You get:

( x y ) ( x + y ) ( x 10 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 10 ) ( x 10 + 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 10 ) (x-y) (x+y) \left(x^{10}-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^{10}\right) \left(x^{10}+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^{10}\right)

Now, plugging in the values of x and y with 6 and 1, you get 5 \boxed{5} and 7 \boxed{7} for the first two factors.

We need another factor:

Consider ( 6 22 1 ) (6^{22}-1) . Note that 22 = 23 -1 and 23 is prime

By Fermat's Little theorem,

6 22 6 23 1 1 ( m o d 23 ) 6^{22}\equiv6^{23-1}\equiv1 \pmod {23}

23 ( 6 22 1 ) \\ \implies 23 | (6^{22} - 1)

So, the other factor is 23 \boxed{23}

TL;DR Go to Wolfram|Alpha and type

FactorInteger[6^22-1]

which gives

{{5, 1}, {7, 1}, {23, 1}, {3154757, 1}, {51828151, 1}}

If you liked my solution, please upvote and tell me what is the correct way to write divides in LaTeX

Use \mid.

Note that you still have to explain why no smaller prime works.

Calvin Lin Staff - 7 years ago

How did u do the factorization x^22-y^22 ?

Chandrachur Banerjee - 6 years, 9 months ago

will 11 be another factor?

Sourayan Banerjee - 7 years ago

Did You use binomial theorem for this factorization?

Krishna Ar - 7 years ago

6²²-1= 5×7×23×3154757×51828151

Carlos Suarez - 6 years, 11 months ago

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Correct! Way of present generation for this particular question.

Lu Chee Ket - 6 years, 10 months ago
Joanne Lee
Jun 7, 2014

We are looking for the smallest three primes m m where 6 22 1 ( m o d m ) 6^{22} \equiv 1\pmod{m} .

Obviously, 2 and 3 are not valid options, so we next look to 5.

It so happens that 6 6 22 1 ( m o d 5 ) 5 6 22 6 \equiv 6^{22} \equiv 1 \pmod{5} \implies 5 \mid 6^{22} , 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 ) 11 1 ( m o d 7 ) 6 \equiv -1 \pmod{7} \implies 6^2 \equiv 1\pmod{7} \implies {(6^2)}^{11} \equiv 1 \pmod{7} 7 6 22 \implies 7 \mid 6^{22} . So we have our second prime.

Note: More generally, if you have a number a n 1 a^n-1 , it would always be divisible by a 1 a-1 , and if n n is an even number, a + 1 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 22 = 6 23 1 1 ( m o d 23 ) 23 6 22 6^{22}=6^{23-1}\equiv 1 \pmod{23} \implies 23\mid 6^{22} .

Therefore, the answer is 5 + 7 + 23 = 35 5+7+23=\boxed{35} .

Why you stoped at three numbers ? why not you add 11 ?

Imane L. Fettochi - 6 years, 10 months ago
Nathan Ramesh
May 27, 2014

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

Finn Hulse - 7 years ago

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I wrote that solution.

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Yes, you did. Thanks! :D

Finn Hulse - 7 years ago

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@Finn Hulse Also you need to bash all primes under 23 23 to make sure that 23 23 is the third smallest. I didn't like that part, it wasn't very elegant.

Daniel Liu - 6 years, 11 months ago
Steven Zheng
Jul 15, 2014

Neat trick. I programmed a python prime factorization algorithm to solve.

Works like a charm.

Steven Zheng - 6 years, 11 months ago

Worst solution ever!

Sai Nikhil Thirandas - 6 years, 10 months ago

Why to use a program every time?Apply your brains

Anuj Shikarkhane - 6 years, 10 months ago

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.

Lu Chee Ket
Aug 7, 2014

= 131621703842267135 = 5 x 7 x 23 x 163505222164307

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