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If p x = y 4 + 4 p^x=y^4+4 , where p p is a prime number and x x and y y are the natural numbers. Then find the value of p + x + y p+x+y .


The answer is 7.

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3 solutions

Kazem Sepehrinia
Aug 22, 2015

p x = y 4 + 4 y 2 + 4 4 y 2 = ( y 2 2 y + 2 ) ( y 2 + 2 y + 2 ) p^x=y^4+4y^2+4-4y^2=(y^2-2y+2)(y^2+2y+2) Now, two factors in the RHS are powers of p p and since y 2 2 y + 2 < y 2 + 2 y + 2 y^2-2y+2<y^2+2y+2 we must have y 2 2 y + 2 y 2 + 2 y + 2 y 2 2 y + 2 y 2 + 2 y + 2 ( y 2 2 y + 2 ) = 4 y y^2-2y+2|y^2+2y+2 \\ y^2-2y+2|y^2+2y+2-(y^2-2y+2)=4y Thus 4 y y 2 2 y + 2 ( y 3 ) 2 7 y 3 2 1 y 5 4y\ge y^2-2y+2 \\ (y-3)^2 \le 7 \\ |y-3|\le 2 \\ 1\le y \le 5 Trying these values in turn shows that only possibility is y = 1 y=1 , which leads to p = 5 p=5 and x = 1 x=1 and p + x + y = 7 p+x+y=7 .

Tried 5^1 = 1^4 +4

Nice logic friend. No need to calculate so much. I have upvoted ur solution.CHEERS

Kaushik Chandra - 4 years, 8 months ago
Soumava Pal
Aug 23, 2015

This problem is a very popular one. Use sophie germaine's identity and show that none of the factors is equal to 1. Thus, we get p=5, y=1, x=1. Besides you can see the proof in Titu's book on Diophantine Equations.

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