\[\begin{vmatrix} a_{11} & a_{12} & a_{13} & ... & a{1,2n-2} & a_{1,2n-1} &a_{1,2n} \\ 0 & a_{22} & a_{23} & ... & a_{2,2n-2} & a_{2,2n-1} & a_{2,2n}\\ 0 & 0 & a_{23} & ... & a_{3,2n-2} & 0 & 0 \\ . & . & . & ... & . & . & . \\ 0 & 0 & a_{2n-2,3} & ... & a_{2n-2,2n-2} & 0 & 0\\ 0 & a_{2n-1,2} & a_{2n-1,3} & ... & a_{2n-1,2n-2} & a_{2n-1,2n-1} & 0\\ a_{2n,1} & a_{2n,2} & a_{2n,3} & ... & a_{2n,2n-2} & a_{2n,2n-2} & a_{2n,2n} \end{vmatrix}\]
