Fourier bases of a class of planar self-affine measures
Let $μ_{M,D}$ be the planar self-affine measure generated by an expansive integer matrix $M\in M_2(\mathbb{Z})$ and a non-collinear integer digit set $D=\left\{\begin{pmatrix} 0\\0\end{pmatrix},\begin{pmatrix} α_{1}\\ α_{2} \end{pmatrix}, \begin{pmatrix} β_{1}\\ β_{2} \end{pmatrix}, \begin{pmatrix} -α_{1}-β_{1}\\ -α_{2}-β_{2} \end{pmatrix}\right\}$. In this paper, we show that $μ_{M,D}$ is a spectral measure if and only if there exists a matrix $Q\in M_2(\mathbb{R})$ such that $(\tilde{M},\tilde{D})$ is admissible, where $\tilde{M}=QMQ^{-1}$ and $\tilde{D}=QD$. In particular, when $α_1β_2-α_2β_1\notin 2\Bbb Z$, $μ_{M,D}$ is a spectral measure if and only if $M\in M_2(2\mathbb{Z})$.