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Algebra Difficulty 7.6 National olympiad, round 2 Find the answer

Does there exist a real 3×33 \times 3 matrix AA such that \operatorname{tr}(\mathrm{A})=0and and A^{2}+A^{t}=I?(tr ? (tr (\mathrm{A})denotesthetraceof denotes the trace of A,, A^{t}isthetransposeof is the transpose of A,and, and I$ 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})=0and and A^{2}+A^{t}=I.Takingthetranspose,wehave. Taking the transpose, we have A=I(A2)t=I(At)2=I(IA2)2=2A2A4A=I-\left(A^{2}\right)^{t}=I-\left(A^{t}\right)^{2}=I-\left(I-A^{2}\right)^{2}=2 A^{2}-A^{4} A42A2+A=0A^{4}-2 A^{2}+A=0Therootsofthepolynomial The roots of the polynomial x^{4}-2 x^{2}+x=x(x-1)\left(x^{2}+x-1\right)are are 0,1, \frac{-1 \pm \sqrt{5}}{2}sothesenumberscanbetheeigenvaluesof so these numbers can be the eigenvalues of A;theeigenvaluesof; the eigenvalues of A^{2}canbe can be 0,1, \frac{1 \pm \sqrt{5}}{2}.Bytr(A)=0. By \operatorname{tr}(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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