Past exam of the year 1 of the computer science course of the University of Oxford / 2022 / Digital systems, linear algebra, introduction to formal proof / 5 / d / i / Solution

Let $f_j$ be the determinant of the leading $j\times j$ principal submatrix. Expansion along the last row gives an $a_j{f}_{j-1}$ term and a $c_{j-1}$ term with negative cofactor sign. In the latter minor, the final column has only $b_{j-1}$ nonzero, and deleting that row and column leaves the leading $(j-2)\times (j-2)$ principal submatrix. Thus the tridiagonal determinant recurrence isHere $f_0=1$ is the empty determinant convention, needed to include $f_2=a_1{a}_2-b_1{c}_1$. The derivation uses cofactor expansion, so the recurrence relation remains valid when coefficients or intermediate determinants vanish; it never divides by a pivot element.