Theorem sb6 2125

Description: Alternate definition of substitution when variables are disjoint. Compare Theorem 6.2 of [Quine] p. 40. Also proved as Lemmas 16 and 17 of [Tarski] p. 70. The implication "to the left" also holds without a disjoint variable condition (sb2 2517). Theorem sb6f 2535 replaces the disjoint variable condition with a nonfreeness hypothesis. Theorem sb4b 2513 replaces it with a distinctor antecedent. (Contributed by NM, 18-Aug-1993.) (Proof shortened by Wolf Lammen, 21-Sep-2018.) Revise df-sb 2098. (Revised by BJ, 22-Dec-2020.) Remove use of ax-11 2198. (Revised by Steven Nguyen, 7-Jul-2023.) (Proof shortened by Wolf Lammen, 16-Jul-2023.)

Assertion

Ref	Expression
sb6	⊢ ([𝑡 / 𝑥]𝜑 ↔ ∀𝑥(𝑥 = 𝑡 → 𝜑))

Distinct variable group:   𝑥,𝑡

Allowed substitution hints:   𝜑(𝑥,𝑡)

Proof of Theorem sb6

Dummy variable 𝑦 is distinct from all other variables.

Step	Hyp	Ref	Expression
1		dfsb 2100	. 2 ⊢ ([𝑡 / 𝑥]𝜑 ↔ ∀𝑦(𝑦 = 𝑡 → ∀𝑥(𝑥 = 𝑦 → 𝜑)))
2		equequ2 2053	. . . . 5 ⊢ (𝑦 = 𝑡 → (𝑥 = 𝑦 ↔ 𝑥 = 𝑡))
3	2	imbi1d 344	. . . 4 ⊢ (𝑦 = 𝑡 → ((𝑥 = 𝑦 → 𝜑) ↔ (𝑥 = 𝑡 → 𝜑)))
4	3	albidv 1947	. . 3 ⊢ (𝑦 = 𝑡 → (∀𝑥(𝑥 = 𝑦 → 𝜑) ↔ ∀𝑥(𝑥 = 𝑡 → 𝜑)))
5	4	equsalvw 2031	. 2 ⊢ (∀𝑦(𝑦 = 𝑡 → ∀𝑥(𝑥 = 𝑦 → 𝜑)) ↔ ∀𝑥(𝑥 = 𝑡 → 𝜑))
6	1, 5	bitri 278	1 ⊢ ([𝑡 / 𝑥]𝜑 ↔ ∀𝑥(𝑥 = 𝑡 → 𝜑))

Syntax hints:   → wi 4   ↔ wb 209  ∀wal 1565  [wsb 2097

This theorem was proved from axioms:  ax-mp 5  ax-1 6  ax-2 7  ax-3 8  ax-gen 1822  ax-4 1836  ax-5 1937  ax-6 1994  ax-7 2035

This theorem depends on definitions:  df-bi 210  df-an 401  df-ex 1807  df-sb 2098

This theorem is referenced by:  2sb6  2126  sb1v  2127  sbrimvwOLD  2132  sbbiiev  2133  sbievwOLD  2135  nfs1v  2197  sb4av  2286  sb6a  2300  sb5  2317  sbievOLD  2354  sb8v  2391  sb8f  2392  2eu6  2690  nfabdw  2952  elab6g  3637  iota4  6518  axregs  35474  in-ax8  36624  mh-setind  36935  regsfromregtco  36937  regsfromsetind  36938  regsfromunir1  36939  bj-df-sb  37160  bj-dfsbc  37162  bj-ax12ssb  37168  bj-sbievwd  37290  bj-hbs1  37335  bj-hbsb2av  37337  bj-sbievw1  37368  bj-sbievw2  37369  bj-sbievw  37370  wl-sbid2ft  38087  wl-sb9v  38091  wl-lem-moexsb  38110  absnsb  47652  ichnfimlem  48100