A problem by Δημήτριος Χαλάτσης

Level pending

Suppose three complex numbers α 0 , α 1 , α 2 α_{0},α_{1},α_{2} such that

(α-2)( α \overline{α} -2)+|α-2|=2. Also suppose the complex number u such that u 3 u^{3} + α 2 u 2 α_{2}u^{2} + α 1 u α_{1}u + α 0 α_{0} =0 . Find the smallest integer n : |u|< n


The answer is 4.

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1 solution

From the gi \ven equality about (α-2)( α \overline{α} -2)+|α-2|=2 <=> |α-2|=1 => |α|<3 then α 2 u 2 α_{2}u^{2} + α 1 u α_{1}u + α 0 α_{0} =0 <=> u 3 u^{3} + α 2 u 2 α_{2}u^{2} + α 1 u α_{1}u + α 0 α_{0} =- u 3 u^{3}
=> | α 2 u 2 α_{2}u^{2} + α 1 u α_{1}u + α 0 α_{0} |=| u 3 u^{3} | through the triangular inequality we have that

| α 2 u 2 α_{2}u^{2} |+| α 1 u α_{1}u |+| α 0 α_{0} | > | u 3 u^{3} | since |α|< 3

| 3 u 2 3u^{2} | + | 3 u 3u | + 3 > | α 2 u 2 α_{2}u^{2} |+| α 1 u α_{1}u |+| α 0 α_{0} | > | u 3 u^{3} | =>

| 3 u 2 3u^{2} | + | 3 u 3u | + 3 > | u 3 u^{3} | <=> | u 3 u^{3} | - | 3 u 2 3u^{2} | - | 3 u 3u | - 3 < 0 using horners method to factorize the "polynomial" with | u u | - 4 (| u u | - 4)( | u 2 u^{2} | + | u u | + 1) + 1 < 0 <=> (| u u | - 4)( | u 2 u^{2} | + | u u | + 1) < -1 => (| u u | - 4)( | u 2 u^{2} | + | u u | + 1) < 0 since ( | u 2 u^{2} | + | u u | + 1) > 0 always | u u | - 4 < 0 <=> | u u | < 4 4

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