2441

What is the largest prime factor of 2 4 4 1 24^4 - 1 ?


The answer is 577.

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16 solutions

Abrar Nihar
Sep 2, 2013

2 4 4 1 ~~~~~~~\Large{24^4-1}

= ( 2 4 2 ) 2 1 2 \Large{=(24^2)^2-1^2}

= ( 2 4 2 + 1 ) ( 2 4 2 1 ) \Large{= (24^2+1)(24^2-1)}

= ( 2 4 2 + 1 ) ( 24 + 1 ) ( 24 1 ) \Large{=(24^2+1)(24+1)(24-1)}

= 577 × 25 × 23 \Large{=577 \times 25 \times 23}

= 5 2 × 23 × 577 \Large{=5^2 \times 23 \times 577}

~

We can see that 577 = 24 \Large{\textrm{We can see that}⌊\sqrt{577}⌋ = 24 }

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Diving 577 by every prime below 24... \Large{\textrm{Diving 577 by every prime below 24...}}

We can find that 577 is a prime \Large{\textrm{We can find that 577 is a prime}}

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Hence, the largest prime is \Large{\textrm{Hence, the largest prime is}} 577 \LARGE{\fbox{577}}

upvote for the friendly font size

Daniel Wang - 7 years, 9 months ago
Russell Few
Sep 1, 2013

2 4 4 1 = ( 2 4 2 + 1 ) ( 2 4 2 1 ) = ( 2 4 2 + 1 ) ( 24 + 1 ) ( 24 1 ) 24^4-1=(24^2+1)(24^2-1)=(24^2+1)(24+1)(24-1) . We would show that 2 4 2 + 1 = 577 24^2+1=577 is a prime.

Note that it is not divisible by 3, 5, 7, 11, 13, 17, 19, or 23. Since 577 < 29 \sqrt{577}<29 , we know now that it is a prime. Hence the answer is 577 \boxed{577} .

We know that 577 is the largest possible prime since the other 2 factors, 25 and 23 are both less than 577.

Russell FEW - 7 years, 9 months ago

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There wasn't a need to factor 2 4 2 1 24^2-1 (though I'd admit it's simple, since 2 4 2 + 1 > 2 4 2 1 24^2 + 1 > 24^2 - 1 , so 577 must be the largest prime.

Calvin Lin Staff - 7 years, 9 months ago

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But what if 577 577 wasn't a prime??? We couldn't be sure whether 2 4 2 + 1 24^2 + 1 is a prime or composite... If 577 577 wasn't a prime, we would need the other prime factors to continue... Prime factorization should make the solution more clear...

Abrar Nihar - 7 years, 9 months ago

@ Calvin: Agreed. I think that in addition to that I should have done all the trial divisions by 3, 5, 7, 11, 13, 17, 19 and 23 to 577.

Russell FEW - 7 years, 9 months ago
Timmy Ben
Sep 2, 2013

From a 2 a^{2} - b 2 b^{2} =( a + b a + b )( a b a - b )

We know

2 4 4 24^{4} - 1 4 1^{4} = ( 2 4 2 24^{2} + 1 2 1^{2} ) ( 2 4 2 24^{2} - 1 2 1^{2} ) = ( 2 4 2 24^{2} + 1 2 1^{2} ) ( 24 + 1 24 + 1 ) ( 24 1 24 - 1 ) = 577 × 25 × 23 577 \times 25 \times 23

Hence, 577, 25 and 23 are factors of 2 4 4 24^{4} - 1 4 1^{4} ,and only 577 and 23 are prime factors, thus the largest prime factor is 577

Vishwa Iyer
Sep 1, 2013

2 4 4 1 = ( 2 4 2 + 1 ) ( 2 4 2 1 ) = ( 577 ) ( 24 + 1 ) ( 24 1 ) = ( 577 ) ( 5 2 ) ( 23 ) \begin{aligned} 24^4 - 1 & = \left(24^2 + 1\right)\left(24^2 - 1\right) \\ & = \left(577\right)\left(24 + 1\right)\left(24 - 1\right) \\ & = \left(577\right)\left(5^2\right)\left(23\right) \end{aligned}

Here is the equation, 577 \boxed{577} is the largest prime, thus it is our answer.

Moderator note:

You should verify that 577 is indeed prime.

Dani Natanael
Sep 2, 2013

= 2 4 4 1 =24^{4}-1

= ( 2 4 2 + 1 ) ( 2 4 2 1 ) =(24^{2}+1)(24^{2}-1)

= ( 2 4 2 + 1 ) ( 24 + 1 ) ( 24 1 ) =(24^{2}+1)(24+1)(24-1)

= ( 577 ) . ( 25 ) . ( 23 ) =(577).(25).(23)

Since 577 is prime, so the largest prime factor is 577

Alexander Sludds
Sep 1, 2013

By repeatedly applying the difference of squares we can see that 2 4 4 1 = ( 2 4 2 1 ) ( 2 4 2 + 1 ) = 23 × 25 × 577 24^{4}-1=(24^{2}-1)(24^{2}+1)=23 \times 25 \times 577 . We need to factor 577 577 and see that if we test all of the primes up to the floor of the square root of 577 577 we get that 577 577 is prime. Thus, 577 577 is our answer.

Ronaldo Dias
Sep 4, 2013

Se Repararmos Isto é um Quadrado da Soma Pela Diferença :

(24^2+1) \times (24^2-1) (24^2+1) \times (24 +1) \times (24-1) (577) \times (5^2) \times (23)

Assim Temos o Maior Primo = 577

Valian Fil Ahli
Sep 2, 2013

let 24=x

x^4-1=(x^2+1)(x^2-1) =(x^2+1)(x+1)(x-1) =(24^2+1)(24+1)(24-1) =(577)(25)(23)

because 577 is prime number,so,the largest prime factor of 24^4-1 is 577

24^4 - 1 = 331776 - 1 = 331775.

the prime factorization of 331775 = 5^2 . 23 . 577.

then the largest prime factor of 24^4 - 1 = 577

wonderfull solution: http://www.mathwarehouse.com/arithmetic/numbers/prime-number/prime-factorization-calculator.php

Andrei Popescu - 7 years, 8 months ago
Aejeth Lord
Nov 5, 2013

Factorize, check whether 577 577 is a prime (by taking its square root and dividing with prime numbers less than its square root) .

Anirudh Sharma
Sep 8, 2013

24^4-1= (24^2+1)(24^2-1). The greater prime factor is 24^2+1= 577.

Gilbert Chia
Sep 7, 2013

Since 24^4 = 576^2, so (576^2)-1 = 575*577. Thus 577 is the largest prime factor of (24^4)-1.

Equinox 123
Sep 6, 2013

24^4 - 1 can be written as (24 ^ 2 - 1) (24 ^ 2 + 1) which can be further broken down as (23) * (25) * (577).

Waldir F. Caro
Sep 5, 2013

24^4 - 1

         = (24^2 - 1) * (24^2 + 1)

         = (24 - 1) * (24 + 1) * (24^2 + 1)

         = 23 * 5^2 * 577

Where the grates prime factor is 577.

Joseph Gomes
Sep 3, 2013

24^4-1=331776-1=331775=5×66355=5×5×13271=5×5×23×577

or, 24^4-1=24^4-1^4=(24^2+1^2)×(24^2-1^2)=(24^2+1^2)×(24-1)×(24+1)=577×23×25=577×23×5×5

so, 577 is the largest prime factor of 24^4-1

again: http://www.mathwarehouse.com/arithmetic/numbers/prime-number/prime-factorization-calculator.php

Andrei Popescu - 7 years, 8 months ago
Toan Pham Quang
Sep 1, 2013

2 4 4 1 = 5 2 577 23 24^4-1=5^2 \cdot 577 \cdot 23

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