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Definition df-ifp 1059
 Description: Definition of the conditional operator for propositions. The expression if-(𝜑, 𝜓, 𝜒) is read "if 𝜑 then 𝜓 else 𝜒". See dfifp2 1060, dfifp3 1061, dfifp4 1062, dfifp5 1063, dfifp6 1064 and dfifp7 1065 for alternate definitions. This definition (in the form of dfifp2 1060) appears in Section II.24 of [Church] p. 129 (Definition D12 page 132), where it is called "conditioned disjunction". Church's [𝜓, 𝜑, 𝜒] corresponds to our if-(𝜑, 𝜓, 𝜒) (note the permutation of the first two variables). This form was chosen as the definition rather than dfifp2 1060 for compatibility with intuitionistic logic development: with this form, it is clear that if-(𝜑, 𝜓, 𝜒) implies decidability of 𝜑, which is most often what is wanted. Church uses the conditional operator as an intermediate step to prove completeness of some systems of connectives. The first result is that the system {if-, ⊤, ⊥} is complete: for the induction step, consider a formula of n+1 variables; single out one variable, say 𝜑; when one sets 𝜑 to True (resp. False), then what remains is a formula of n variables, so by the induction hypothesis it is equivalent to a formula using only the connectives if-, ⊤, ⊥, say 𝜓 (resp. 𝜒); therefore, the formula if-(𝜑, 𝜓, 𝜒) is equivalent to the initial formula of n+1 variables. Now, since { → , ¬ } and similar systems suffice to express the connectives if-, ⊤, ⊥, they are also complete. (Contributed by BJ, 22-Jun-2019.)
Assertion
Ref Expression
df-ifp (if-(𝜑, 𝜓, 𝜒) ↔ ((𝜑𝜓) ∨ (¬ 𝜑𝜒)))

Detailed syntax breakdown of Definition df-ifp
StepHypRef Expression
1 wph . . 3 wff 𝜑
2 wps . . 3 wff 𝜓
3 wch . . 3 wff 𝜒
41, 2, 3wif 1058 . 2 wff if-(𝜑, 𝜓, 𝜒)
51, 2wa 399 . . 3 wff (𝜑𝜓)
61wn 3 . . . 4 wff ¬ 𝜑
76, 3wa 399 . . 3 wff 𝜑𝜒)
85, 7wo 844 . 2 wff ((𝜑𝜓) ∨ (¬ 𝜑𝜒))
94, 8wb 209 1 wff (if-(𝜑, 𝜓, 𝜒) ↔ ((𝜑𝜓) ∨ (¬ 𝜑𝜒)))
 Colors of variables: wff setvar class This definition is referenced by:  dfifp2  1060  dfifp6  1064  ifpor  1068  casesifp  1074  ifpbi123dOLD  1076  1fpid3  1079  wlk1walk  27426  upgriswlk  27428  bj-df-ifc  33987  bj-dfif  33988  bj-ififc  33989  wl-ifpimpr  34842  ifpdfan  40104  ifpnot23  40116  upgrwlkupwlk  44307
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