Past exam of the year 1 of the computer science course of the University of Oxford / 2014 / Digital systems, linear algebra, introduction to formal proof / 6 / c

Solve two triangular linear systems, rather than forming an inverse matrix:For this LU decomposition, forward substitution gives $y_1=b_1$, $y_2=b_2+2y_1$ and $y_3=b_3-3y_1+y_2$. Then back substitution givesAll diagonal entries used as divisors are nonzero. The same LU decomposition can be reused for many right-hand vectors, with only these two triangular solves repeated.