Definition df-ifp 1007

Description: Definition of the conditional operator for propositions. The value of if-(𝜑, 𝜓, 𝜒) is 𝜓 if 𝜑 is true and 𝜒 if 𝜑 false. See dfifp2 1008, dfifp3 1009, dfifp4 1010, dfifp5 1011, dfifp6 1012 and dfifp7 1013 for alternate definitions.

This definition (in the form of dfifp2 1008) 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).

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 wff with n+1 variables; single out one variable, say 𝜑; when one sets 𝜑 to True (resp. False), then what remains is a wff of n variables, so by the induction hypothesis it corresponds to a formula using only {if-, ⊤, ⊥}, say 𝜓 (resp. 𝜒); therefore, the formula if-(𝜑, 𝜓, 𝜒) represents the initial wff. Now, since { → , ¬ } and similar systems suffice to express if-, ⊤, ⊥, they are also complete.

(Contributed by BJ, 22-Jun-2019.)

Assertion

Ref	Expression
df-ifp	⊢ (if-(𝜑, 𝜓, 𝜒) ↔ ((𝜑 ∧ 𝜓) ∨ (¬ 𝜑 ∧ 𝜒)))

Detailed syntax breakdown of Definition df-ifp

Step	Hyp	Ref	Expression
1		wph	. . 3 wff 𝜑
2		wps	. . 3 wff 𝜓
3		wch	. . 3 wff 𝜒
4	1, 2, 3	wif 1006	. 2 wff if-(𝜑, 𝜓, 𝜒)
5	1, 2	wa 383	. . 3 wff (𝜑 ∧ 𝜓)
6	1	wn 3	. . . 4 wff ¬ 𝜑
7	6, 3	wa 383	. . 3 wff (¬ 𝜑 ∧ 𝜒)
8	5, 7	wo 382	. 2 wff ((𝜑 ∧ 𝜓) ∨ (¬ 𝜑 ∧ 𝜒))
9	4, 8	wb 195	1 wff (if-(𝜑, 𝜓, 𝜒) ↔ ((𝜑 ∧ 𝜓) ∨ (¬ 𝜑 ∧ 𝜒)))

This definition is referenced by:  dfifp2  1008  dfifp6  1012  ifpor  1015  casesifp  1020  ifpbi123d  1021  1fpid3  1023  bj-df-ifc  31735  ifpdfan  36829  ifpnot23  36842  1wlk1walk  40843  upgr1wlkwlk  40847