Past exam of the year 1 of the computer science course of the University of Oxford / 2016 / Digital systems, linear algebra, introduction to formal proof (resit) / 1 / d

Only now may the invalid codes 10–15 be marked as don't-care conditions. The $f$ Karnaugh map givesThe groups are $\{8,\dots ,15\}$, $\{4,5,12,13\}$, $\{4,6,12,14\}$ and $\{0,4,8,12\}$. They cover all required ones, avoid valid-digit zeros, and may assign either output to invalid codes. The valid inputs 0, 5, 6 and 9 cannot share a valid implicant with each other, so at least four product terms are necessary. The displayed four-term cover uses only seven literals.