Past exam of the year 1 of the computer science course of the University of Oxford / 2003 / Calculus, probability, linear algebra, discrete mathematics, logic and proof, and models of computation / 4 / b / i / Solution

Let $A_n$ denote the given tridiagonal matrix and put $D_n=\det A_n$, including $D_0=1$ for the empty determinant. The starting values areFor $n\ge 3$, apply the following elementary row operations in the indicated order, always using the updated rows:The first three rows now beginwith the fourth column omitted if $n=3$. When $n\ge 4$, also use $R_4\leftarrow R_4-R_3$. The resulting block upper triangular matrix has a $3\times 3$ upper triangular matrix with diagonal entries all one in its top left corner, and $A_{n-3}$ in its bottom right corner. All elementary row operations preserved the determinant except the single row interchange. ThereforeApplying this recurrence relation to the three starting values proves by mathematical induction thatEquivalently, the sequence is periodic with period six, with values $1,1,0,-1,-1,0$ starting at $n=0$. A compact closed form is $D_n=\sin ((n+1)\pi /3)/\sin (\pi /3)$.