Integer solution?

How many integers a a and b b satisfy the equation below? n = 0 6 ( a + n ) 3 = n = 0 1 ( b + n ) 4 \sum_{n=0}^6( a+n)^{3}=\sum_{n=0}^1(b+n)^{4}


The answer is 0.

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2 solutions

Aaghaz Mahajan
May 16, 2018

Look in mod 7...........

Advait Nene
Apr 28, 2020

The numbers a , a + 1 , a + 2 , a + 3 , a + 4 , a + 5 , a,a+1,a+2,a+3,a+4,a+5, and a + 6 a+6 make a complete set of residues modulo 7 7 . Therefore, when we add up their cubes, it is the same as adding 0 3 + 1 3 + 2 3 + 3 3 + 4 3 + 5 3 + 6 3 0 ( m o d 7 ) 0^{3}+1^{3}+2^{3}+3^{3}+4^{3}+5^{3}+6^{3}\equiv 0\pmod{7} Now consider the sum of two consecutive fourth powers modulo 7 7 . The fourth powers modulo 7 7 are 0 , 1 , 2 , 4 , 4 , 2 , 0,1,2,4,4,2, and 1 1 . There are no two consecutive fourth powers that add to 0 ( m o d 7 ) 0\pmod{7} , so there are no solutions.

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