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Friday, November 8, 2024

2024/052) Solve in integers x^3+3367=2^n

We are given x^3+3367=2^n

We know $3367=7 * 13 &*37

So let is work x^3=2^n \pmod 7

Working in mod 7 we have x^3 \in \{1,-1,0\} and  2^n \in \{1,2,4\} so we get 1 as common and for that n has to be multiple of 3.

So we get

x^3= 2^{3k} \pmod 7

Going back to the original equation we get

x^3 + 3367 = 2^{3k}

Or (2^k)^3 - x^3 = 3367

Or y^3 -x^3 = 3367 where y = 2^k

As (y-x) | y^3-x^3 so y-x |  3367\cdots(1)

Further 

As we know (y-x)^3 = y^3-3y^2x + 3yx^2 - x^3 = (y^3-x^3) - 3yx(y-x) \lt y^3-x^3 when y-x \gt 0

So y-x\lt 15\cdots(2)

So y-x \in \{1,7\}\cdots(2) from (1) and (2)  

 y-x =1

gives x^3 + 3367 = (x+1)^3 = x^3 + 3x^2 + 3x + 1

Or 3367 = 3x^2+ 3x + 1

Or 3x^2+3x = 3366

Or x^2 + x = 1122

x = 33 and y = 34 and y is not power of 2 so not a solution

y-x=7

gives

x^3 + 3367 = x^3 + 21x^2 + 147 x + 343

or $3024 = 21x^2+ 147x

or x^2+ 7x = 144

or x = 9 or x=-16

only positive value admissible giving x = 9 and y = 16

2^n = y^3 giving 2^n = 2^{12} or n = 12

Hence x=3 , n = 12

 

 


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