# I keep getting mixed up on whether the diagonalized form of A is T^(-1)AT or TAT^(-1). Is there an easy way to remember the correct form?

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This is the way I use to remember which form to use. Note the $$T\,$$ matrix consists of all the eigenvectors, $$v_i\,$$, of $$A\,$$$T=(v_1|v_2|\cdots|v_n)\,$. Using the definition of (right) eigenvectors$Av_i=\lambda_i v_i\,$, we have$AT = A(v_1|v_2|\cdots|v_n) = (Av_1|Av_2|\cdots|Av_n) = (\lambda_1v_1|\lambda_2v_2|\cdots|\lambda_nv_n) \,$ $$= (v_1|v_2|\cdots|v_n)\begin{pmatrix} \lambda_1 \\ & \lambda_2 \\ & & \ddots \\ & & & \lambda_n \end{pmatrix}=T\Lambda \,$$
Right multiply each side by $$T^{-1}\,$$$A=T\Lambda T^{-1}\,$, which is the correct form.