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Cube hunting (Posted on 2020-05-29) Difficulty: 3 of 5
Find the set of integers n such that n6 + 24n3 + 192 is a perfect cube.

No Solution Yet Submitted by Danish Ahmed Khan    
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Solution Solution | Comment 6 of 9 |
Define f(n) = n^6+24n^3+192

The simplest solutions would be when f(n) = n^6.  Trying that implies 24n^3+192 = 0, which makes n=-2 and f(n)=64 one solution.

Lets make a compound inequality (n^2-1)^3 < f(n) < (n^2+1)^3.  The only perfect cube between (n^2-1)^3 and (n^2+1)^3 is n^6.  If there are any integer solutions when f(n) != n^6 then n must fail the compound inequality.

(n^2-1)^3 < f(n) is false for -7.999<=n<=-2.171 and f(n) < (n^2+1)^3 is false for -1.831<=n<=7.999.  Combining these then limits integer n to the range -7 to 7.  No new solutions are in this range so set of integer n for which f(n) is a perfect cube is a single value n=-2.

  Posted by Brian Smith on 2020-05-30 07:18:47
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