27 lines
490 B
Plaintext
27 lines
490 B
Plaintext
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import Mathlib.Data.Set.Basic
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/-! # Enderton.Set.Chapter_4
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Natural Numbers
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-/
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namespace Enderton.Set.Chapter_4
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/-- #### Theorem 4C
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Every natural number except `0` is the successor of some natural number.
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-/
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theorem theorem_4c (n : ℕ)
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: n = 0 ∨ (∃ m : ℕ, n = m.succ) := by
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match n with
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| 0 => simp
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| m + 1 => simp
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/-- #### Exercise 4.1
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Show that `1 ≠ 3` i.e., that `∅⁺ ≠ ∅⁺⁺⁺`.
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-/
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theorem exercise_4_1 : 1 ≠ 3 := by
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simp
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end Enderton.Set.Chapter_4
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