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Mirrors > Home > MPE Home > Th. List > Mathboxes > bj-gl4 | Structured version Visualization version GIF version |
Description: In a normal modal logic, the modal axiom GL implies the modal axiom (4). Note that the antecedent of bj-gl4 31753 is an instance of the axiom GL, with 𝜑 replaced by (∀𝑥𝜑 ∧ 𝜑), sometimes called the "strong necessity" of 𝜑. (Contributed by BJ, 12-Dec-2019.) (Proof modification is discouraged.) (New usage is discouraged.) |
Ref | Expression |
---|---|
bj-gl4 | ⊢ ((∀𝑥(∀𝑥(∀𝑥𝜑 ∧ 𝜑) → (∀𝑥𝜑 ∧ 𝜑)) → ∀𝑥(∀𝑥𝜑 ∧ 𝜑)) → (∀𝑥𝜑 → ∀𝑥∀𝑥𝜑)) |
Step | Hyp | Ref | Expression |
---|---|---|---|
1 | bj-gl4lem 31752 | . . 3 ⊢ (∀𝑥𝜑 → ∀𝑥(∀𝑥(∀𝑥𝜑 ∧ 𝜑) → (∀𝑥𝜑 ∧ 𝜑))) | |
2 | 19.26 1786 | . . . 4 ⊢ (∀𝑥(∀𝑥𝜑 ∧ 𝜑) ↔ (∀𝑥∀𝑥𝜑 ∧ ∀𝑥𝜑)) | |
3 | 2 | biimpi 205 | . . 3 ⊢ (∀𝑥(∀𝑥𝜑 ∧ 𝜑) → (∀𝑥∀𝑥𝜑 ∧ ∀𝑥𝜑)) |
4 | 1, 3 | imim12i 60 | . 2 ⊢ ((∀𝑥(∀𝑥(∀𝑥𝜑 ∧ 𝜑) → (∀𝑥𝜑 ∧ 𝜑)) → ∀𝑥(∀𝑥𝜑 ∧ 𝜑)) → (∀𝑥𝜑 → (∀𝑥∀𝑥𝜑 ∧ ∀𝑥𝜑))) |
5 | simpl 472 | . 2 ⊢ ((∀𝑥∀𝑥𝜑 ∧ ∀𝑥𝜑) → ∀𝑥∀𝑥𝜑) | |
6 | 4, 5 | syl6 34 | 1 ⊢ ((∀𝑥(∀𝑥(∀𝑥𝜑 ∧ 𝜑) → (∀𝑥𝜑 ∧ 𝜑)) → ∀𝑥(∀𝑥𝜑 ∧ 𝜑)) → (∀𝑥𝜑 → ∀𝑥∀𝑥𝜑)) |
Colors of variables: wff setvar class |
Syntax hints: → wi 4 ∧ wa 383 ∀wal 1473 |
This theorem was proved from axioms: ax-mp 5 ax-1 6 ax-2 7 ax-3 8 ax-gen 1713 ax-4 1728 |
This theorem depends on definitions: df-bi 196 df-an 385 |
This theorem is referenced by: (None) |
Copyright terms: Public domain | W3C validator |