Metamath Proof Explorer


Theorem welb

Description: A nonempty subset of a well-ordered set has a lower bound. (Contributed by Jeff Madsen, 2-Sep-2009)

Ref Expression
Assertion welb ( ( 𝑅 We 𝐴 ∧ ( 𝐵𝐶𝐵𝐴𝐵 ≠ ∅ ) ) → ( 𝑅 Or 𝐵 ∧ ∃ 𝑥𝐵 ( ∀ 𝑦𝐵 ¬ 𝑥 𝑅 𝑦 ∧ ∀ 𝑦𝐵 ( 𝑦 𝑅 𝑥 → ∃ 𝑧𝐵 𝑦 𝑅 𝑧 ) ) ) )

Proof

Step Hyp Ref Expression
1 wess ( 𝐵𝐴 → ( 𝑅 We 𝐴𝑅 We 𝐵 ) )
2 1 impcom ( ( 𝑅 We 𝐴𝐵𝐴 ) → 𝑅 We 𝐵 )
3 weso ( 𝑅 We 𝐵𝑅 Or 𝐵 )
4 2 3 syl ( ( 𝑅 We 𝐴𝐵𝐴 ) → 𝑅 Or 𝐵 )
5 cnvso ( 𝑅 Or 𝐵 𝑅 Or 𝐵 )
6 4 5 sylib ( ( 𝑅 We 𝐴𝐵𝐴 ) → 𝑅 Or 𝐵 )
7 6 3ad2antr2 ( ( 𝑅 We 𝐴 ∧ ( 𝐵𝐶𝐵𝐴𝐵 ≠ ∅ ) ) → 𝑅 Or 𝐵 )
8 wefr ( 𝑅 We 𝐵𝑅 Fr 𝐵 )
9 2 8 syl ( ( 𝑅 We 𝐴𝐵𝐴 ) → 𝑅 Fr 𝐵 )
10 9 3ad2antr2 ( ( 𝑅 We 𝐴 ∧ ( 𝐵𝐶𝐵𝐴𝐵 ≠ ∅ ) ) → 𝑅 Fr 𝐵 )
11 ssidd ( 𝐵𝐴𝐵𝐵 )
12 11 3anim2i ( ( 𝐵𝐶𝐵𝐴𝐵 ≠ ∅ ) → ( 𝐵𝐶𝐵𝐵𝐵 ≠ ∅ ) )
13 12 adantl ( ( 𝑅 We 𝐴 ∧ ( 𝐵𝐶𝐵𝐴𝐵 ≠ ∅ ) ) → ( 𝐵𝐶𝐵𝐵𝐵 ≠ ∅ ) )
14 frinfm ( ( 𝑅 Fr 𝐵 ∧ ( 𝐵𝐶𝐵𝐵𝐵 ≠ ∅ ) ) → ∃ 𝑥𝐵 ( ∀ 𝑦𝐵 ¬ 𝑥 𝑅 𝑦 ∧ ∀ 𝑦𝐵 ( 𝑦 𝑅 𝑥 → ∃ 𝑧𝐵 𝑦 𝑅 𝑧 ) ) )
15 10 13 14 syl2anc ( ( 𝑅 We 𝐴 ∧ ( 𝐵𝐶𝐵𝐴𝐵 ≠ ∅ ) ) → ∃ 𝑥𝐵 ( ∀ 𝑦𝐵 ¬ 𝑥 𝑅 𝑦 ∧ ∀ 𝑦𝐵 ( 𝑦 𝑅 𝑥 → ∃ 𝑧𝐵 𝑦 𝑅 𝑧 ) ) )
16 7 15 jca ( ( 𝑅 We 𝐴 ∧ ( 𝐵𝐶𝐵𝐴𝐵 ≠ ∅ ) ) → ( 𝑅 Or 𝐵 ∧ ∃ 𝑥𝐵 ( ∀ 𝑦𝐵 ¬ 𝑥 𝑅 𝑦 ∧ ∀ 𝑦𝐵 ( 𝑦 𝑅 𝑥 → ∃ 𝑧𝐵 𝑦 𝑅 𝑧 ) ) ) )