How To Study As A Mathematics Major
《How To Study As A Mathematics Major》 · 第 31 页 · Section 0.9.3「Triangular matrices」，定义段
A matrix $T=[t_{ij}]\in M_{n,m}(F)$ is upper triangular if $t_{ij}=0$ whenever $i>j$. Analogously, $T$ is lower triangular if its transpose is upper triangular. A triangular matrix is either lower or upper triangular.