How many integers a and b satisfy the equation below? n = 0 ∑ 6 ( a + n ) 3 = n = 0 ∑ 1 ( b + n ) 4
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The numbers a , a + 1 , a + 2 , a + 3 , a + 4 , a + 5 , and a + 6 make a complete set of residues modulo 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 ) Now consider the sum of two consecutive fourth powers modulo 7 . The fourth powers modulo 7 are 0 , 1 , 2 , 4 , 4 , 2 , and 1 . There are no two consecutive fourth powers that add to 0 ( m o d 7 ) , so there are no solutions.
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Look in mod 7...........