A number theory problem by Marcos Araujo

The number N = 2 33 2 19 2 17 1 N=2^{33} - 2^{19} - 2^{17} - 1 has exactly one divisor between 1000 and 5000. Which divisor is that?


The answer is 1983.

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4 solutions

Marcos Araujo
Aug 1, 2014

This is my soluction for the problem:

A A , B B and C C are the roots of x 3 ( A + B + C ) X 2 + ( A B + A C + B C ) x + A B C = 0 x^3 -(A + B + C)X^2 + (AB+AC+BC)x + ABC = 0

So, when we make x = A x = A , x = B x = B and x = C x = C the sentence is true:

A 3 ( A + B + C ) A 2 + ( A B + A C + B C ) A A B C = 0 A^3 -(A + B + C)A^2 + (AB+AC+BC)A - ABC = 0 (I)

B 3 ( A + B + C ) B 2 + ( A B + A C + B C ) B A B C = 0 B^3 -(A + B + C)B^2 + (AB+AC+BC)B - ABC = 0 (II)

C 3 ( A + B + C ) C 2 + ( A B + A C + B C ) C A B C = 0 C^3 -(A + B + C)C^2 + (AB+AC+BC)C - ABC = 0 (III)

Adding (I) + (II) + (III) we get:

( A 3 + B 3 + C 3 ) 3 A B C = ( A + B + C ) ( A 2 + B 2 + C 2 ) + ( A + B + C ) ( A B + A C + B C ) (A^3+B^3+C^3) - 3ABC = (A + B + C)(A^2 + B^2 + C^2) + (A + B + C)(AB+AC+BC)

( A 3 + B 3 + C 3 ) 3 A B C = ( A + B + C ) ( A 2 + B 2 + C 2 + A B + A C + B C ) (A^3+B^3+C^3) - 3ABC = (A + B + C)(A^2+B^2+C^2 +AB + AC + BC)

This is a well known factorization.

Now lets make:

2 33 = ( 2 11 ) 3 2^{33} = \left(2^{11}\right)^3 ;

2 19 = 2 × 2 18 = 2 18 2 18 = ( 2 6 ) 3 2 × 2 17 -2^{19} =-2\times 2^{18}= -2^{18} -2^{18} = (-2^6)^3 - 2\times 2^{17}

2 17 = 2 11 × 2 6 -2^{17} = -2^{11}\times 2^6

Now lets look the original expression:

2 33 2 19 2 17 1 2^{33} - 2^{19} - 2^{17} - 1

and lets re write it as:

( 2 11 ) 3 + ( 2 6 ) 3 + ( 1 3 ) 3 × 2 11 × ( 2 6 ) × ( 1 ) \left(2^{11}\right)^3 +(-2^6)^3 +(-1^3) - 3\times 2^{11}\times (-2^6) \times (-1)

Making A = 2 11 ; B = 2 6 A = 2^{11}; B = -2^6 and C = 1 C = -1 we get:

( 2 11 ) 3 + ( 2 6 ) 3 + ( 1 3 ) 3 × 2 11 × ( 2 6 ) × ( 1 ) \left(2^{11}\right)^3 +(-2^6)^3 +(-1^3) - 3\times2^{11}\times (-2^6) \times (-1) = ( 2 11 2 6 1 ) ( A 2 + B 2 + C 2 A B A C B C ) = (2^{11} - 2^6 - 1)(A^2+B^2+C^2 -AB - AC - BC) = 1983 × ( A 2 + B 2 + C 2 A B A C B C ) = 1983\times(A^2+B^2+C^2 -AB - AC - BC)

So 1983 is the wanted divisor.

Thanks for all who shared and liked :-)

Rudresh Tomar
Jul 16, 2014

its factors are: 3^(3) * 13 * 661 * 37021

so the answer => 3*661=1983

using calculator?? I did that way actually :p

BTW, the problem maker must be a cricket lover and an Indian... ;)

Kartik Sharma - 6 years, 11 months ago

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What! You checked all the numbers from 1000 to 5000??

Anuj Shikarkhane - 6 years, 10 months ago

????? please clarify

أبو مصعب - 6 years, 11 months ago

I can't understand ?

Khush Romana - 6 years, 11 months ago

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Since its the first time India won world cup

ashutosh mahapatra - 6 years, 10 months ago

Isn't this a number theory problem?

Steven Zheng - 6 years, 11 months ago
1
2
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N = 2**33 - 2**19 - 2**17 - 1
for i in range(1001,5000):
    if N%i == 0:
        print i

Mehul Chaturvedi
Oct 10, 2014

1, 3, 9, 13, 27, 39, 117, 351, 661, 1983, 5949, 8593, 17847, 25779, 37021, 77337, 111063, 232011, 333189, 481273,

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