Diophantine equation-problem 1

x y y x = x y 2 19 x^y-y^x=xy^2-19

Solve the equation above for primes x x and y y . Given that there are two solution pairs ( x 1 , y 1 ) (x_1, y_1) and ( x 2 , y 2 ) (x_2, y_2) , find the value of ( x 1 + y 1 ) ( x 2 + y 2 ) (x_1+y_1)(x_2 + y_2) .


The answer is 45.

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2 solutions

Nitin Kumar
Feb 15, 2020

Widening out (a lot) from primes to all integers, there still are very few solutions to this equation. Just these two and ( x , y ) = ( 1 , 5 ) or ( 1 , 4 ) (x,y) = (1, -5) \text { or } (1, 4) , I think.

Matthew Feig - 1 year, 3 months ago

how are there only 2 solutions sounds a little weird

Nitin Kumar - 1 year, 3 months ago

Did you conjecture this or prove it

Nitin Kumar - 1 year, 3 months ago

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I do not have a full proof. Basically, if x y x \neq y , then unless both variables are quite 'small,' one of the first two terms (whichever has the greater exponent) dominates all the other terms and quickly prevents equality. So the search range for positive integer solutions can be restricted to a small set (eg. both x x and y y must be less than 10 10 ), and then checked by brute force.

Consider the case of y > x 3 y > x \geq 3 : 3 4 4 3 = 17 3^4 - 4^3 = 17 , 3 5 5 3 = 118 3^5 - 5^3 = 118 , 4 5 5 4 = 399 4^5 - 5^4 = 399 , ... Once this difference becomes greater than y 3 > x y 2 > x y 2 19 y^3 > xy^2 > xy^2 - 19 , there is no need to check for further solutions. It is pretty well known that x y > y x x^y > y^x for y > x y > x once both variables are greater than e e , so greater than 3 3 in the case of integers. Because of the terms on the right hand side, you have to check a bit farther than 3 3 . I think it would be sufficient to check until x y > 2 y x x^y > 2y^x for y > x y > x . This is where I need some help.

The cases of x = 1 , 2 x = 1,2 or y = 1 , 2 y=1,2 can be handled separately, which end up leading to some solutions.

Matthew Feig - 1 year, 3 months ago

ok, i see] well thought

Nitin Kumar - 1 year, 3 months ago

What did you do with the graph?

Nitin Kumar - 1 year, 3 months ago

I plotted the solutions of f(x,y) =-19

A Former Brilliant Member - 1 year, 3 months ago

oh the graph was a little confusing. Now I understood

Nitin Kumar - 1 year, 3 months ago

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