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Get all the Real Parts of the Problem (Posted on 2016-12-03) Difficulty: 4 of 5
Generalize the equation from get the real part of the problem to become
x^8 - b*x^4 + c = 0.

For which integers n (from 0 to 8, obviously) is it possible to choose coefficients b and c such that there are n roots in which the real portion is positive. The preceding problem shows n=3 is possible with (b,c)=(-1,-240)

  Submitted by Brian Smith    
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Solution: (Hide)
n can be 2, 3 or 4.

Factor the polynomial into (x^4 - r1)*(x^4 - r2). Then the eight roots consist of the four complex fourth roots of r1 and the four complex fourth roots of r2.

Each set of four roots will have either 1 or 2 roots with positive real parts; 1 in the case of r1 or r2 being a positive real or 2 in any other case. Then {1 or 2} + {1 or 2} yields {2, 3, or 4} possible roots of the original equaiton that have a real part.

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