feat: port Algebra.ContinuedFractions.Translations (#3783)

Co-authored-by: Moritz Firsching <firsching@google.com>

Mathlib.lean

@@ -52,6 +52,7 @@ import Mathlib.Algebra.CharZero.Infinite
 import Mathlib.Algebra.CharZero.Lemmas
 import Mathlib.Algebra.CharZero.Quotient
 import Mathlib.Algebra.ContinuedFractions.Basic
+import Mathlib.Algebra.ContinuedFractions.Translations
 import Mathlib.Algebra.CovariantAndContravariant
 import Mathlib.Algebra.CubicDiscriminant
 import Mathlib.Algebra.DirectSum.Basic

Mathlib/Algebra/ContinuedFractions/Translations.lean

@@ -0,0 +1,192 @@
+/-
+Copyright (c) 2019 Kevin Kappelmann. All rights reserved.
+Released under Apache 2.0 license as described in the file LICENSE.
+Authors: Kevin Kappelmann
+
+! This file was ported from Lean 3 source module algebra.continued_fractions.translations
+! leanprover-community/mathlib commit a7e36e48519ab281320c4d192da6a7b348ce40ad
+! Please do not edit these lines, except to modify the commit id
+! if you have ported upstream changes.
+-/
+import Mathlib.Algebra.ContinuedFractions.Basic
+
+/-!
+# Basic Translation Lemmas Between Functions Defined for Continued Fractions
+
+## Summary
+
+Some simple translation lemmas between the different definitions of functions defined in
+`Algebra.ContinuedFractions.Basic`.
+-/
+
+
+namespace GeneralizedContinuedFraction
+
+section General
+
+/-!
+### Translations Between General Access Functions
+
+Here we give some basic translations that hold by definition between the various methods that allow
+us to access the numerators and denominators of a continued fraction.
+-/
+
+
+variable {α : Type _} {g : GeneralizedContinuedFraction α} {n : ℕ}
+
+theorem terminatedAt_iff_s_terminatedAt : g.TerminatedAt n ↔ g.s.TerminatedAt n := by rfl
+#align generalized_continued_fraction.terminated_at_iff_s_terminated_at GeneralizedContinuedFraction.terminatedAt_iff_s_terminatedAt
+
+theorem terminatedAt_iff_s_none : g.TerminatedAt n ↔ g.s.get? n = none := by rfl
+#align generalized_continued_fraction.terminated_at_iff_s_none GeneralizedContinuedFraction.terminatedAt_iff_s_none
+
+theorem part_num_none_iff_s_none : g.partialNumerators.get? n = none ↔ g.s.get? n = none := by
+  cases s_nth_eq : g.s.get? n <;> simp [partialNumerators, s_nth_eq]
+#align generalized_continued_fraction.part_num_none_iff_s_none GeneralizedContinuedFraction.part_num_none_iff_s_none
+
+theorem terminatedAt_iff_part_num_none : g.TerminatedAt n ↔ g.partialNumerators.get? n = none := by
+  rw [terminatedAt_iff_s_none, part_num_none_iff_s_none]
+#align generalized_continued_fraction.terminated_at_iff_part_num_none GeneralizedContinuedFraction.terminatedAt_iff_part_num_none
+
+theorem part_denom_none_iff_s_none : g.partialDenominators.get? n = none ↔ g.s.get? n = none := by
+  cases s_nth_eq : g.s.get? n <;> simp [partialDenominators, s_nth_eq]
+#align generalized_continued_fraction.part_denom_none_iff_s_none GeneralizedContinuedFraction.part_denom_none_iff_s_none
+
+theorem terminatedAt_iff_part_denom_none : g.TerminatedAt n ↔ g.partialDenominators.get? n = none :=
+  by rw [terminatedAt_iff_s_none, part_denom_none_iff_s_none]
+#align generalized_continued_fraction.terminated_at_iff_part_denom_none GeneralizedContinuedFraction.terminatedAt_iff_part_denom_none
+
+theorem part_num_eq_s_a {gp : Pair α} (s_nth_eq : g.s.get? n = some gp) :
+    g.partialNumerators.get? n = some gp.a := by simp [partialNumerators, s_nth_eq]
+#align generalized_continued_fraction.part_num_eq_s_a GeneralizedContinuedFraction.part_num_eq_s_a
+
+theorem part_denom_eq_s_b {gp : Pair α} (s_nth_eq : g.s.get? n = some gp) :
+    g.partialDenominators.get? n = some gp.b := by simp [partialDenominators, s_nth_eq]
+#align generalized_continued_fraction.part_denom_eq_s_b GeneralizedContinuedFraction.part_denom_eq_s_b
+
+theorem exists_s_a_of_part_num {a : α} (nth_part_num_eq : g.partialNumerators.get? n = some a) :
+    ∃ gp, g.s.get? n = some gp ∧ gp.a = a := by
+  simpa [partialNumerators, Stream'.Seq.map_get?] using nth_part_num_eq
+#align generalized_continued_fraction.exists_s_a_of_part_num GeneralizedContinuedFraction.exists_s_a_of_part_num
+
+theorem exists_s_b_of_part_denom {b : α}
+    (nth_part_denom_eq : g.partialDenominators.get? n = some b) :
+    ∃ gp, g.s.get? n = some gp ∧ gp.b = b := by
+  simpa [partialDenominators, Stream'.Seq.map_get?] using nth_part_denom_eq
+#align generalized_continued_fraction.exists_s_b_of_part_denom GeneralizedContinuedFraction.exists_s_b_of_part_denom
+
+end General
+
+section WithDivisionRing
+
+/-!
+### Translations Between Computational Functions
+
+Here we  give some basic translations that hold by definition for the computational methods of a
+continued fraction.
+-/
+
+
+variable {K : Type _} {g : GeneralizedContinuedFraction K} {n : ℕ} [DivisionRing K]
+
+theorem nth_cont_eq_succ_nth_cont_aux : g.continuants n = g.continuantsAux (n + 1) :=
+  rfl
+#align generalized_continued_fraction.nth_cont_eq_succ_nth_cont_aux GeneralizedContinuedFraction.nth_cont_eq_succ_nth_cont_aux
+
+theorem num_eq_conts_a : g.numerators n = (g.continuants n).a :=
+  rfl
+#align generalized_continued_fraction.num_eq_conts_a GeneralizedContinuedFraction.num_eq_conts_a
+
+theorem denom_eq_conts_b : g.denominators n = (g.continuants n).b :=
+  rfl
+#align generalized_continued_fraction.denom_eq_conts_b GeneralizedContinuedFraction.denom_eq_conts_b
+
+theorem convergent_eq_num_div_denom : g.convergents n = g.numerators n / g.denominators n :=
+  rfl
+#align generalized_continued_fraction.convergent_eq_num_div_denom GeneralizedContinuedFraction.convergent_eq_num_div_denom
+
+theorem convergent_eq_conts_a_div_conts_b :
+    g.convergents n = (g.continuants n).a / (g.continuants n).b :=
+  rfl
+#align generalized_continued_fraction.convergent_eq_conts_a_div_conts_b GeneralizedContinuedFraction.convergent_eq_conts_a_div_conts_b
+
+theorem exists_conts_a_of_num {A : K} (nth_num_eq : g.numerators n = A) :
+    ∃ conts, g.continuants n = conts ∧ conts.a = A := by simpa
+#align generalized_continued_fraction.exists_conts_a_of_num GeneralizedContinuedFraction.exists_conts_a_of_num
+
+theorem exists_conts_b_of_denom {B : K} (nth_denom_eq : g.denominators n = B) :
+    ∃ conts, g.continuants n = conts ∧ conts.b = B := by simpa
+#align generalized_continued_fraction.exists_conts_b_of_denom GeneralizedContinuedFraction.exists_conts_b_of_denom
+
+@[simp]
+theorem zeroth_continuant_aux_eq_one_zero : g.continuantsAux 0 = ⟨1, 0⟩ :=
+  rfl
+#align generalized_continued_fraction.zeroth_continuant_aux_eq_one_zero GeneralizedContinuedFraction.zeroth_continuant_aux_eq_one_zero
+
+@[simp]
+theorem first_continuant_aux_eq_h_one : g.continuantsAux 1 = ⟨g.h, 1⟩ :=
+  rfl
+#align generalized_continued_fraction.first_continuant_aux_eq_h_one GeneralizedContinuedFraction.first_continuant_aux_eq_h_one
+
+@[simp]
+theorem zeroth_continuant_eq_h_one : g.continuants 0 = ⟨g.h, 1⟩ :=
+  rfl
+#align generalized_continued_fraction.zeroth_continuant_eq_h_one GeneralizedContinuedFraction.zeroth_continuant_eq_h_one
+
+@[simp]
+theorem zeroth_numerator_eq_h : g.numerators 0 = g.h :=
+  rfl
+#align generalized_continued_fraction.zeroth_numerator_eq_h GeneralizedContinuedFraction.zeroth_numerator_eq_h
+
+@[simp]
+theorem zeroth_denominator_eq_one : g.denominators 0 = 1 :=
+  rfl
+#align generalized_continued_fraction.zeroth_denominator_eq_one GeneralizedContinuedFraction.zeroth_denominator_eq_one
+
+@[simp]
+theorem zeroth_convergent_eq_h : g.convergents 0 = g.h := by
+  simp [convergent_eq_num_div_denom, num_eq_conts_a, denom_eq_conts_b, div_one]
+#align generalized_continued_fraction.zeroth_convergent_eq_h GeneralizedContinuedFraction.zeroth_convergent_eq_h
+
+theorem second_continuant_aux_eq {gp : Pair K} (zeroth_s_eq : g.s.get? 0 = some gp) :
+    g.continuantsAux 2 = ⟨gp.b * g.h + gp.a, gp.b⟩ := by
+  simp [zeroth_s_eq, continuantsAux, nextContinuants, nextDenominator, nextNumerator]
+#align generalized_continued_fraction.second_continuant_aux_eq GeneralizedContinuedFraction.second_continuant_aux_eq
+
+theorem first_continuant_eq {gp : Pair K} (zeroth_s_eq : g.s.get? 0 = some gp) :
+    g.continuants 1 = ⟨gp.b * g.h + gp.a, gp.b⟩ := by
+  simp [nth_cont_eq_succ_nth_cont_aux]
+  -- porting note: simp used to work here, but now it can't figure out that 1 + 1 = 2
+  convert second_continuant_aux_eq zeroth_s_eq
+#align generalized_continued_fraction.first_continuant_eq GeneralizedContinuedFraction.first_continuant_eq
+
+theorem first_numerator_eq {gp : Pair K} (zeroth_s_eq : g.s.get? 0 = some gp) :
+    g.numerators 1 = gp.b * g.h + gp.a := by simp [num_eq_conts_a, first_continuant_eq zeroth_s_eq]
+#align generalized_continued_fraction.first_numerator_eq GeneralizedContinuedFraction.first_numerator_eq
+
+theorem first_denominator_eq {gp : Pair K} (zeroth_s_eq : g.s.get? 0 = some gp) :
+    g.denominators 1 = gp.b := by simp [denom_eq_conts_b, first_continuant_eq zeroth_s_eq]
+#align generalized_continued_fraction.first_denominator_eq GeneralizedContinuedFraction.first_denominator_eq
+
+@[simp]
+theorem zeroth_convergent'_aux_eq_zero {s : Stream'.Seq <| Pair K} :
+    convergents'Aux s 0 = (0 : K) :=
+  rfl
+#align generalized_continued_fraction.zeroth_convergent'_aux_eq_zero GeneralizedContinuedFraction.zeroth_convergent'_aux_eq_zero
+
+@[simp]
+theorem zeroth_convergent'_eq_h : g.convergents' 0 = g.h := by simp [convergents']
+#align generalized_continued_fraction.zeroth_convergent'_eq_h GeneralizedContinuedFraction.zeroth_convergent'_eq_h
+
+theorem convergents'Aux_succ_none {s : Stream'.Seq (Pair K)} (h : s.head = none) (n : ℕ) :
+    convergents'Aux s (n + 1) = 0 := by simp [convergents'Aux, h, convergents'Aux.match_1]
+#align generalized_continued_fraction.convergents'_aux_succ_none GeneralizedContinuedFraction.convergents'Aux_succ_none
+
+theorem convergents'Aux_succ_some {s : Stream'.Seq (Pair K)} {p : Pair K} (h : s.head = some p)
+    (n : ℕ) : convergents'Aux s (n + 1) = p.a / (p.b + convergents'Aux s.tail n) := by
+  simp [convergents'Aux, h, convergents'Aux.match_1]
+#align generalized_continued_fraction.convergents'_aux_succ_some GeneralizedContinuedFraction.convergents'Aux_succ_some
+
+end WithDivisionRing
+
+end GeneralizedContinuedFraction