AlgebraDifficulty 7.6National olympiad, round 2Find the answer
Does there exist a real 3×3 matrix A such that \operatorname{tr}(\mathrm{A})=0andA^{2}+A^{t}=I?(tr(\mathrm{A})denotesthetraceofA,A^{t}isthetransposeofA,andI$ is the identity matrix.)
A number or a short expression. Spacing and $ signs are ignored.
Solution
The answer is NO. Suppose that \operatorname{tr}(\mathrm{A})=0andA^{2}+A^{t}=I.Takingthetranspose,wehaveA=I−(A2)t=I−(At)2=I−(I−A2)2=2A2−A4A4−2A2+A=0Therootsofthepolynomialx^{4}-2 x^{2}+x=x(x-1)\left(x^{2}+x-1\right)are0,1, \frac{-1 \pm \sqrt{5}}{2}sothesenumberscanbetheeigenvaluesofA;theeigenvaluesofA^{2}canbe0,1, \frac{1 \pm \sqrt{5}}{2}.Bytr(A)=0, the sum of the eigenvalues is 0 , and by \operatorname{tr}\left(A^{2}\right)=\operatorname{tr}\left(I-A^{t}\right)=3$ the sum of squares of the eigenvalues is 3 . It is easy to check that this two conditions cannot be satisfied simultaneously.
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