Triplets of Integers

How many triplets of positive integers ( a , b , c ) (a,b,c) exist such that a 6 + b 2 + c 3 = 2018 a^6+b^2+c^3 = 2018 ?


The answer is 4.

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1 solution

There would be 3 3 options for a a , for if a > 3 a>3 , then a 6 > 2018 a^6>2018 .

Also note that a square, mod 3 3 , can either be 0 0 or 1 1 .

If a = 1 a=1 , then b 2 + c 3 = 2017 b 2 + c 3 1 ( m o d 3 ) b^2+c^3=2017 \implies b^2+c^3 \equiv 1 \ (mod \ 3) . therefore, we cannot have c = 3 k + 2 c=3k+2 , for k k being a non-negative integer. So, we need to check 8 8 possibilities for 0 < c < ( 2017 ) 1 3 0<c< (2017)^{\frac{1}{3}} . There is only one solution here ( 1 , 17 , 12 ) (1,17,12) .

For a = 2 a=2 , we have b 2 + c 3 = 1954 b^2+c^3=1954 and we need to check odd c c in the range 0 < c < ( 1954 ) 1 3 0<c<(1954)^{\frac{1}{3}} . there would be 6 6 possibilities and, at the end, the would be one solution ( 2 , 35 , 9 ) (2,35,9) .

For a = 3 a=3 , b 2 + c 3 = 1289 b^2+c^3=1289 . There are 10 10 possibilities for 0 < c < ( 1289 ) 1 3 0<c<(1289)^{\frac{1}{3}} . Two solution emerges eventually ( 3 , 17 , 10 ) (3,17,10) and ( 3 , 35 , 4 ) (3,35,4) .

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