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Algebra Difficulty 7.7 National Olympiad, round 2 Find the answer

Determine all positive integers nn for which there exist n×nn \times n real invertible matrices AA and BB that satisfy ABBA=B2AA B-B A=B^{2} A.

A number or a short expression. Fractions can be typed as 3/2, and spacing doesn't matter.

Solution

We prove that there exist such matrices AA and BB if and only if nn is even. I. Assume that nn is odd and some invertible n×nn \times n matrices A,BA, B satisfy ABBA=B2AA B-B A=B^{2} A. Hence B=A1(B2+B)AB=A^{-1}\left(B^{2}+B\right) A, so the matrices BB and B2+BB^{2}+B are similar and therefore have the same eigenvalues. Since nn is odd, the matrix BB has a real eigenvalue, denote it by λ1\lambda_{1}. Therefore λ2:=λ12+λ1\lambda_{2}:=\lambda_{1}^{2}+\lambda_{1} is an eigenvalue of B2+BB^{2}+B, hence an eigenvalue of BB. Similarly, λ3:=λ22+λ2\lambda_{3}:=\lambda_{2}^{2}+\lambda_{2} is an eigenvalue of B2+BB^{2}+B, hence an eigenvalue of BB. Repeating this process and taking into account that the number of eigenvalues of BB is finite we will get there exist numbers klk \leq l so that λl+1=λk\lambda_{l+1}=\lambda_{k}. Hence λk+1=λk2+λkλl=λl12+λl1λk=λl2+λl\lambda_{k+1} =\lambda_{k}^{2}+\lambda_{k} \ldots \lambda_{l} =\lambda_{l-1}^{2}+\lambda_{l-1} \lambda_{k} =\lambda_{l}^{2}+\lambda_{l} Adding these equations we get λk2+λk+12++λl2=0\lambda_{k}^{2}+\lambda_{k+1}^{2}+\ldots+\lambda_{l}^{2}=0. Taking into account that all λi\lambda_{i} 's are real (as λ1\lambda_{1} is real), we have λk==λl=0\lambda_{k}=\ldots=\lambda_{l}=0, which implies that BB is not invertible, contradiction. II. Now we construct such matrices A,BA, B for even nn. Let A2=[0110]A_{2}=\left[\begin{array}{ll}0 & 1 \\ 1 & 0\end{array}\right] and B2=[1111]B_{2}=\left[\begin{array}{cc}-1 & 1 \\ -1 & -1\end{array}\right]. It is easy to check that the matrices A2,B2A_{2}, B_{2} are invertible and satisfy the condition. For n=2kn=2 k the n×nn \times n block matrices A=[A2000A2000A2],B=[B2000B2000B2]A=\left[\begin{array}{cccc} A_{2} & 0 & \ldots & 0 \\ 0 & A_{2} & \ldots & 0 \\ \vdots & \vdots & \ddots & \vdots \\ 0 & 0 & \ldots & A_{2} \end{array}\right], \quad B=\left[\begin{array}{cccc} B_{2} & 0 & \ldots & 0 \\ 0 & B_{2} & \ldots & 0 \\ \vdots & \vdots & \ddots & \vdots \\ 0 & 0 & \ldots & B_{2} \end{array}\right] are also invertible and satisfy the condition.

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