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Axiom ax-luk3 33556
Description: 3 of 3 axioms for propositional calculus due to Lukasiewicz. Copy of luk-3 1737 and pm2.24 122, but introduced as an axiom. One might think that the similar pm2.21 121 𝜑 → (𝜑𝜓)) is a valid replacement for this axiom. But this is not true, ax-3 8 is not derivable from this modification. This can be shown by designing carefully operators ¬ and on a finite set of primitive statements. In propositional logic such statements are and , but we can assume more and other primitives in our universe of statements. So we denote our primitive statements as phi0 , phi1 and phi2. The actual meaning of the statements are not important in this context, it rather counts how they behave under our operations ¬ and , and which of them we assume to hold unconditionally (phi1, phi2). For our disproving model, I give that information in tabular form below. The interested reader may check per hand, that all possible interpretations of ax-mp 5, ax-luk1 33554, ax-luk2 33555 and pm2.21 121 result in phi1 or phi2, meaning they always hold. But for wl-ax3 33568 we can find a counter example resulting in phi0, not a statement always true. The verification of a particular set of axioms in a given model is tedious and error prone, so I wrote a computer program, first checking this for me, and second, hunting for a counter example. Here is the result, after 9165 fruitlessly computer generated models:

ax-3 fails for phi2, phi2
number of statements: 3
always true phi1 phi2

Negation is defined as
-. phi0-. phi1-. phi2

Implication is defined as
p->qq: phi0q: phi1q: phi2
p: phi0phi1phi1phi1
p: phi1phi0phi1phi1
p: phi2phi0phi0phi0

(Contributed by Wolf Lammen, 17-Dec-2018.) (New usage is discouraged.)
Ref Expression
ax-luk3 (𝜑 → (¬ 𝜑𝜓))

Detailed syntax breakdown of Axiom ax-luk3
StepHypRef Expression
1 wph . 2 wff 𝜑
21wn 3 . . 3 wff ¬ 𝜑
3 wps . . 3 wff 𝜓
42, 3wi 4 . 2 wff 𝜑𝜓)
51, 4wi 4 1 wff (𝜑 → (¬ 𝜑𝜓))
Colors of variables: wff setvar class
This axiom is referenced by:  wl-con4i  33562  wl-pm2.24i  33563  wl-ax3  33568  wl-ax1  33569  wl-pm2.21  33572  wl-id  33578
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