- Thread starter
- Admin
- #1

- Feb 14, 2012

- 3,894

-----

Determine all positive integers $x,\,n$ that satisfying the equation $x^3+3367=2^n$.

-----

Remember to read the POTW submission guidelines to find out how to submit your answers!

- Thread starter anemone
- Start date

- Status
- Not open for further replies.

- Thread starter
- Admin
- #1

- Feb 14, 2012

- 3,894

-----

Determine all positive integers $x,\,n$ that satisfying the equation $x^3+3367=2^n$.

-----

Remember to read the POTW submission guidelines to find out how to submit your answers!

- Thread starter
- Admin
- #2

- Feb 14, 2012

- 3,894

Now 3367 = 7 * 13 *37

so let us work mod 7

$x^3 = 2^n \pmod 7$

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

So we have $x^3 = 2^{3m} \pmod 7$

Going back to the original equation we have

$x^3 + 3367= 2^{3m} \pmod 7$

or $(2^m)^3 - x^3 = 3367$'

We can put $y= 2^m$ to keep it simple

So $y^3-x^3 = 3367$

as $(y-x) | (y^3-x^3$

so y = x = 1 or 7 or 13 or 37

as $(y-x)^2 = (y^2 + x^2 - 2xy) < y^2 + xy + x^2 $ so we have

$(y-x) = 1$ or 7 or 13

By checking y - x = 1 and 13 we do not get any root (method as as below but not mentioned and y-x = 7 gives root as y = 16, x = 9 as below

$ y - x = 7\cdots(1)$

so $y^2 + x^2 + xy = 481$

or $(y-x)^2 + 3xy = 481$

or $7^2 + 3xy = 481$

or $xy = 144\cdots(2)$

From (1) and (2) we get $y=16, x = 9$ or $n=12,x = 9$

- Status
- Not open for further replies.