It will be 2017 or not

Given the equation

4 x 3 + 7 y 3 = 2017 \large \color{#D61F06}{4x^{3}}+\color{#3D99F6}{7y^{3}}=\color{#69047E}{2017}

How many pairs of positive integers ( x , y ) (x,y) that satisfy the equation?

3 2 0 5 1 4

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3 solutions

Skye Rzym
Mar 11, 2017

We know that 2017 1 ( m o d 7 ) 2017\equiv 1 \pmod 7 . So, we can conclude that 4 x 3 + 7 y 3 1 ( m o d 7 ) 4x^{3}+7y^{3}\equiv 1 \pmod 7 4 x 3 1 ( m o d 7 ) 4x^{3}\equiv 1 \pmod 7 x 3 2 ( m o d 7 ) x^{3}\equiv 2 \pmod 7 For x 0 , 1 , 2 , 3 , 4 , 5 , 6 ( m o d 7 ) x\equiv 0,1,2,3,4,5,6 \pmod 7 . There will be x 3 0 , 1 , 6 ( m o d 7 ) x^{3}\equiv 0,1,6 \pmod 7 . So, there are no solution in postive integers for x x . And we can conclude that there are no pairs of positive integers ( x , y ) (x,y)

Mayank Jha
Mar 5, 2017

Since RHS is 3 modulo 4 so must be LHS.So substituting y=4k-1 we transform the equation into x^3+112k^3+21k-84k^2=506.Now since RHS is 2 modulo 7 so must be LHS.But in lhs except x^3 every term is divisible by 7.So x^3 is 2 modulo 7 which is not possible since cubic residues of 7 are 0,1,and 6.

Prithwish Roy
Mar 4, 2017

Well I don't know if its correct or not, but I did it this way:- 4 x 3 + 7 y 3 = 2017 4x^{3} + 7y^{3}=2017 ( 4 1 / 3 x ) 3 + ( 7 1 / 3 y ) 3 = 2017 (4^{1/3}x)^3 + (7^{1/3}y)^{3}=2017

Now as 2017 is prime, the x + y x+y term in x 3 + y 3 x^{3}+y^{3} must be equal to 1 1 . But ( 4 1 / 3 x + 7 1 / 3 y ) (4^{1/3}x+7^{1/3}y) can never be equal to 1 1 for non negative values of x and y. Thus there are 0 \boxed{0} solutions

I really do not mean to be rude, but this is hilarious.

Shourya Pandey - 4 years, 3 months ago

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Care to elaborate?

Prithwish Roy - 4 years, 3 months ago

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