įstrižainė

įstrižanė, kvadratinės matricos $A=\left(\begin{array}{c}\begin{array}{c}{a}_{11}\\ a_{21}\end{array}\\ \mathrm{...}\\ a_{n1}\end{array}\begin{array}{c}\begin{array}{c}{a}_{12}\\ a_{22}\end{array}\\ \mathrm{...}\\ a_{n2}\end{array}\begin{array}{c}\begin{array}{c}\mathrm{...}\\ \mathrm{...}\end{array}\\ \mathrm{...}\\ \mathrm{...}\end{array}\begin{array}{c}\begin{array}{c}{a}_{1,n-1}\\ a_{2,n-1}\end{array}\\ \mathrm{...}\\ a_{n,n-1}\end{array}\begin{array}{c}\begin{array}{c}{a}_{1n}\\ a_{2n}\end{array}\\ \mathrm{...}\\ a_{\mathit{nn}}\end{array}\right)=$(a_ij)_n × n elementai a₁₁, a₂₂, …, a_nn arba elementai a_1n, a_{2, n–1}, …, a_n1. Pirmuoju atveju įstrižainė vadinama pagrindine, antruoju – šalutine.