Legend 2 x 2^{x}

Determine all pairs ( x , y ) (x, y) of integers satisfying the equation: 1 + 2 x + 2 2 x + 1 = y 2 1+2^{x}+2^{2x+1}=y^{2} If your answers are ( x 1 ; y 1 ) , ( x 2 ; y 2 ) , , ( x n ; y n ) (x_{1};y_{1}),(x_{2};y_{2}),\cdot\cdot\cdot,(x_{n};y_{n}) type it as y 1 x 1 + y 2 x 2 + + y n x n y_{1}^{x_{1}}+y_{2}^{x_{2}}+ \cdot\cdot\cdot +y_{n}^{x_{n}}

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The answer is 559684.

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2 solutions

Leonelo Román
Apr 27, 2016

not (4,+-31) but (4,+-23)

I Gede Arya Raditya Parameswara - 3 years, 6 months ago
Hana Wehbi
Jun 10, 2018

We notice that 1 + 2 x + 2 2 x + 1 1+2^x+2^{2x+1} is not an integer if x < 1 x<-1 and equals 2 2 and 4 4 when x = 1 and 0 x= -1 \text{ and } 0 respectively.

Thus, the only solution is when x 0 x\le 0 is ( x , y ) = ( 0 , ± 2 ) (x,y)=(0,\pm2) .

If x 1 x\ge 1 , then 1 + 2 x + 2 2 x + 1 1+2^x+2^{2x+1} is odd. Hence 8 ( y 2 1 ) = 2 x ( 2 x + 1 + 1 ) 8|(y^2-1) = 2^x(2^{x+1} +1) , so that x 3 x\ge 3 .

From ( y 1 ) ( y + 1 ) = 2 x ( 2 x + 1 + 1 ) (y-1)(y+1) = 2^x(2^{x+1}+1) and the gcd ( y 1 , y + 1 ) = 2 \gcd(y-1,y+1)=2 since they are consecutive integers,

we get y 1 2 × y + 1 2 = 2 x 2 ( 2 x + 2 + 1 ) = k ( 8 k + 1 ) \frac{y-1}{2}\times\frac{y+1}{2}= 2^{x-2}(2^{x+2}+1)=k(8k+1) , where k = 2 x 2 k=2^{x-2} is a positive power of 2.

Since y 1 2 \frac{y-1}{2} and y + 1 2 \frac{y+1}{2} are consecutive integers, there must exist positive odd integers r , s r,s such that { y 1 2 , y + 1 2 } = { k r , s } \big\{ \frac{y-1}{2},\frac{y+1}{2}\big\}=\big\{kr,s\big\} and r s = 8 k + 1 rs=8k+1 .

Since 1 = k r s = k r 8 k + 1 r 1=|kr-s| = |kr -\frac{8k+1}{r}| , we must have r = 3 r=3 , Hence k 1 = 3 |k-1|=3 and 2 x 2 = k = 4. 2^{x-2}=k=4. Thus, x = 4 x=4 and y = ± 23. y=\pm 23.

The only solutions in integer pairs are ( x , y ) are ( 0 , ± 2 ) and ( 4 , ± 23 ) y 1 x 1 + y 2 x 2 + + y n x n = 2 0 + ( 2 ) 0 + 2 3 4 + ( 23 ) 4 = 559684 (x,y) \text { are } (0, \pm 2) \text{ and } (4,\pm 23)\implies y_{1}^{x_{1}}+y_{2}^{x_{2}}+ \cdot\cdot\cdot +y_{n}^{x_{n}}= 2^0+(-2)^0+23^4+(-23)^4=\boxed{559684} .

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