Definition df-if 3181

Description: Define the conditional operator. Read if(ph, A, B) as "if ph then A else B." See iftrue 3185 and iffalse 3186 for its values. In mathematical literature, this operator is rarely defined formally but is implicit in informal definitions such as "let f(x)=0 if x=0 and 1/x otherwise." (In older versions of this database, this operator was denoted "ded" and called the "deduction class.")

An important use for us is in conjunction with the weak deduction theorem, which converts a hypothesis into an antecedent. In that role, A is a class variable in the hypothesis and B is a class (usually a constant) that makes the hypothesis true when it is substituted for A. See dedth 3208 for the main part of the weak deduction theorem, elimhyp 3215 to eliminate a hypothesis, and keephyp 3221 to keep a hypothesis. See the Deduction Theorem link on the Metamath Proof Explorer Home Page for a description of the weak deduction theorem.

Assertion

Ref	Expression
df-if	|- if(ph, A, B) = {x | ((x e. A /\ ph) \/ (x e. B /\ -. ph))}

Distinct variable groups:   ph,x   x,A   x,B

Detailed syntax breakdown of Definition df-if

Step	Hyp	Ref	Expression
1		wph	. . 3 wff ph
2		cA	. . 3 class A
3		cB	. . 3 class B
4	1, 2, 3	cif 3180	. 2 class if(ph, A, B)
5		vx	. . . . . . 7 set x
6	5	cv 1585	. . . . . 6 class x
7	6, 2	wcel 1588	. . . . 5 wff x e. A
8	7, 1	wa 337	. . . 4 wff (x e. A /\ ph)
9	6, 3	wcel 1588	. . . . 5 wff x e. B
10	1	wn 2	. . . . 5 wff -. ph
11	9, 10	wa 337	. . . 4 wff (x e. B /\ -. ph)
12	8, 11	wo 336	. . 3 wff ((x e. A /\ ph) \/ (x e. B /\ -. ph))
13	12, 5	cab 2128	. 2 class {x | ((x e. A /\ ph) \/ (x e. B /\ -. ph))}
14	4, 13	wceq 1586	1 wff if(ph, A, B) = {x | ((x e. A /\ ph) \/ (x e. B /\ -. ph))}

This definition is referenced by:  dfif2 3182  iffalse 3186

Copyright terms: Public domain