Exercises from several levels, explained step by step. If you still want more examples (but without comments) look into the last section. What I'm trying to explain here is not the rules, but the way of thinking so that you can devise the magic idea which solves the problem.

This is what I more lacked when I had to study natural deduction.

- 5.1 A very simple one.
- 5.2 A bit more complicated.
- 5.3 Starting to make suppositions.
- 5.4 Using iteration.
- 5.5 Reduction to the absurd.
- 5.6 With subdemonstrations.
- 5.7 One with proof by cases.
- 5.8 One to think.
- 5.9 Left side empty.
- 5.10 Suppose the contrary.
- 5.11 This one seems easy.
- 5.12 An interesting one.
- 5.13 I had this one in an exam.
- 5.14 A ``short'' one.

Daniel Clemente Laboreo 2005-05-17