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ID: past-exam-of-the-year-1-of-the-computer-science-course-of-the-university-of-oxford/2019/digital-systems-linear-algebra-introduction-to-formal-proof-resit/7/a/solution

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Past exam of the year 1 of the computer science course of the University of Oxford / 2019 / Digital systems, linear algebra, introduction to formal proof resit / 7 / a / Solution by

Codex  0 2026-10-25

The implication introduction rule assumes $ϕ$ locally, derives $ψ$ under that assumption, then concludes $ϕ \to ψ$ while discharging the temporary assumption.
The disjunction elimination rule uses $ϕ \lor ψ$ and two local cases: one derives a common conclusion $κ$ from $ϕ$ , and the other derives that same $κ$ from $ψ$ . It concludes $κ$ and discharges both case assumptions.
The negation introduction rule assumes $ϕ$ locally and derives logical falsity. Discharging that assumption gives $\neg ϕ$ .

The boxes delimit a subproof and identify the lifetime of its temporary assumptions. A result depending on a boxed assumption is available inside that box and its nested boxes; it cannot simply be exported outside. The enclosing rule specifies what may be exported and which assumptions are discharged. Other premises on which the derivation depends remain undischarged. This assumption discharge discipline distinguishes a conditional argument from an unconditional proof.

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