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Complex numbers as a field #21
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/- | ||
Copyright (c) 2017 Kevin Buzzard. All rights reserved. | ||
Released under Apache 2.0 license as described in the file LICENSE. | ||
Author: Kevin Buzzard | ||
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The complex numbers, modelled as R^2 in the obvious way. | ||
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TODO: Add topology, and prove that the complexes are a topological ring. | ||
-/ | ||
import analysis.real | ||
noncomputable theory | ||
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-- I am unclear about whether I should be proving that | ||
-- C is a field or a discrete field. As far as I am personally | ||
-- concerned these structures are the same. | ||
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local attribute [instance] classical.decidable_inhabited classical.prop_decidable | ||
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structure complex : Type := | ||
(re : ℝ) (im : ℝ) | ||
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notation `ℂ` := complex | ||
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namespace complex | ||
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theorem eta (z : complex) : complex.mk z.re z.im = z := | ||
cases_on z (λ _ _, rfl) | ||
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theorem eq_of_re_eq_and_im_eq : ∀ (z w : complex), z.re=w.re → z.im=w.im → z=w | ||
| ⟨zr, zi⟩ ⟨wr, wi⟩ rfl rfl := rfl | ||
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-- simp version | ||
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theorem eq_iff_re_eq_and_im_eq (z w : complex) : z=w ↔ z.re=w.re ∧ z.im=w.im := | ||
⟨λ H, ⟨by rw [H],by rw [H]⟩,λ H, eq_of_re_eq_and_im_eq _ _ H.left H.right⟩ | ||
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-- begin | ||
--split, | ||
-- intro H,rw [H],split;trivial, | ||
--exact eq_of_re_eq_and_im_eq _ _, | ||
--end | ||
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lemma proj_re (r i : real) : (complex.mk r i).re = r := rfl | ||
lemma proj_im (r i : real) : (complex.mk r i).im = i := rfl | ||
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local attribute [simp] eq_iff_re_eq_and_im_eq proj_re proj_im | ||
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def of_real (r : ℝ) : ℂ := { re := r, im := 0 } | ||
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protected def zero := of_real 0 | ||
protected def one := of_real 1 | ||
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instance coe_real_complex : has_coe ℝ ℂ := ⟨of_real⟩ | ||
instance has_zero_complex : has_zero complex := ⟨of_real 0⟩ | ||
instance has_one_complex : has_one complex := ⟨of_real 1⟩ | ||
instance inhabited_complex : inhabited complex := ⟨complex.zero⟩ | ||
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@[simp] lemma coe_re (r:real) : ((↑r):complex).re = r := rfl | ||
@[simp] lemma coe_im (r:real) : ((↑r):complex).im = 0 := rfl | ||
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def I : complex := {re := 0, im := 1} | ||
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def conjugate (z : complex) : complex := {re := z.re, im := -(z.im)} | ||
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def norm_squared : complex → real := | ||
λ z, z.re*z.re+z.im*z.im | ||
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protected def inv : complex → complex := | ||
λ z, { re := z.re / norm_squared z, | ||
im := -z.im / norm_squared z } | ||
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instance : has_add complex := ⟨λ z w, { re :=z.re+w.re, im:=z.im+w.im}⟩ | ||
instance : has_neg complex := ⟨λ z, { re := -z.re, im := -z.im}⟩ | ||
instance : has_mul complex := ⟨λ z w, { re := z.re*w.re - z.im*w.im, | ||
im := z.re*w.im + z.im*w.re}⟩ | ||
instance : has_inv complex := ⟨λ z, { re := z.re / norm_squared z, | ||
im := -z.im / norm_squared z }⟩ | ||
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@[simp] lemma zero_re : (0:complex).re=0 := rfl | ||
@[simp] lemma zero_im : (0:complex).im=0 := rfl | ||
@[simp] lemma one_re : (1:complex).re=1 := rfl | ||
@[simp] lemma one_im : (1:complex).im=0 := rfl | ||
@[simp] lemma I_re : complex.I.re=0 := rfl | ||
@[simp] lemma I_im : complex.I.im=1 := rfl | ||
@[simp] lemma conj_re (z : complex) : (conjugate z).re = z.re := rfl | ||
@[simp] lemma conj_im (z : complex) : (conjugate z).im = -z.im := rfl | ||
@[simp] lemma add_re (z w: complex) : (z+w).re=z.re+w.re := rfl | ||
@[simp] lemma add_im (z w: complex) : (z+w).im=z.im+w.im := rfl | ||
@[simp] lemma neg_re (z: complex) : (-z).re=-z.re := rfl | ||
@[simp] lemma neg_im (z: complex) : (-z).im=-z.im := rfl | ||
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-- one size fits all tactic | ||
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meta def crunch : tactic unit := do | ||
`[intros], | ||
`[rw [eq_iff_re_eq_and_im_eq]], | ||
`[split;simp [add_mul,mul_add,mul_assoc]] | ||
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meta def crunch2 : tactic unit := do | ||
`[simp [add_mul, mul_add, eq_iff_re_eq_and_im_eq] {contextual:=tt}] | ||
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meta def crunch3 : tactic unit := do | ||
`[simp [add_mul, mul_add, mul_comm, mul_assoc, eq_iff_re_eq_and_im_eq] {contextual:=tt}] | ||
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instance : add_comm_group complex := | ||
{ add_comm_group . | ||
zero := 0, | ||
add := (+), | ||
neg := has_neg.neg, | ||
zero_add := by crunch, | ||
add_zero := by crunch, | ||
add_comm := by crunch, | ||
add_assoc := by crunch, | ||
add_left_neg := by crunch | ||
} | ||
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@[simp] lemma sub_re (z w : complex) : (z-w).re=z.re-w.re := rfl | ||
@[simp] lemma sub_im (z w : complex) : (z-w).im=z.im-w.im := rfl | ||
@[simp] lemma mul_re (z w: complex) : (z*w).re=z.re*w.re-z.im*w.im := rfl | ||
@[simp] lemma mul_im (z w: complex) : (z*w).im=z.re*w.im+z.im*w.re := rfl | ||
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lemma norm_squared_pos_of_nonzero (z : complex) (H : z ≠ 0) : norm_squared z > 0 := | ||
begin -- far more painful than it should be but I need it for inverses | ||
There was a problem hiding this comment. Choose a reason for hiding this commentThe reason will be displayed to describe this comment to others. Learn more. You are right that this is overcomplicated. I'll clean it up after merge |
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suffices : z.re ≠ 0 ∨ z.im ≠ 0, | ||
{ apply lt_of_le_of_ne, | ||
{ exact add_nonneg (mul_self_nonneg _) (mul_self_nonneg _) }, | ||
intro H2, | ||
cases this with Hre Him, | ||
{ exact Hre (eq_zero_of_mul_self_add_mul_self_eq_zero (eq.symm H2)) }, | ||
unfold norm_squared at H2,rw [add_comm] at H2, | ||
exact Him (eq_zero_of_mul_self_add_mul_self_eq_zero (eq.symm H2)) }, | ||
have : ¬ (z.re = 0 ∧ z.im = 0), | ||
{ intro H2, | ||
exact H (eq_of_re_eq_and_im_eq z 0 H2.left H2.right) }, | ||
by_cases z0 : (z.re = 0),-- with Hre_eq,-- Hre_ne, | ||
{ right, | ||
intro H2, | ||
apply this, | ||
exact ⟨z0,H2⟩ }, | ||
{ left,assumption } | ||
end | ||
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lemma of_real_injective : function.injective of_real := | ||
begin | ||
intros x₁ x₂ H, | ||
exact congr_arg complex.re H, | ||
end | ||
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lemma of_real_zero : of_real 0 = (0:complex) := rfl | ||
lemma of_real_one : of_real 1 = (1:complex) := rfl | ||
lemma of_real_eq_coe (r : real) : of_real r = ↑r := rfl | ||
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lemma of_real_neg (r : real) : -(r:complex) = ((-r:ℝ):complex) := by crunch | ||
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lemma of_real_add (r s: real) : (r:complex) + (s:complex) = ((r+s):complex) := by crunch | ||
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lemma of_real_sub (r s:real) : (r:complex) - (s:complex) = ((r-s):complex) := by crunch | ||
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lemma of_real_mul (r s:real) : (r:complex) * (s:complex) = ((r*s):complex) := by crunch | ||
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lemma of_real_inv (r:real) : (r:complex)⁻¹ = ((r⁻¹):complex) := | ||
begin | ||
There was a problem hiding this comment. Choose a reason for hiding this commentThe reason will be displayed to describe this comment to others. Learn more.
There was a problem hiding this comment. Choose a reason for hiding this commentThe reason will be displayed to describe this comment to others. Learn more. Also: indentation after |
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rw [eq_iff_re_eq_and_im_eq], | ||
split, | ||
unfold has_inv.inv, | ||
unfold has_inv.inv, | ||
end | ||
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lemma of_real_abs_squared (r:real) : norm_squared (of_real r) = (abs r)*(abs r) := | ||
begin | ||
There was a problem hiding this comment. Choose a reason for hiding this commentThe reason will be displayed to describe this comment to others. Learn more. This is an interesting proof: But it only works if There was a problem hiding this comment. Choose a reason for hiding this commentThe reason will be displayed to describe this comment to others. Learn more. Aah! So this is a very good way of explaining in concrete terms the issue that came up earlier. Thanks! |
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rw [abs_mul_abs_self], | ||
suffices : r*r+0*0=r*r, | ||
There was a problem hiding this comment. Choose a reason for hiding this commentThe reason will be displayed to describe this comment to others. Learn more. This |
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exact this, | ||
simp, | ||
end | ||
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#print field | ||
There was a problem hiding this comment. Choose a reason for hiding this commentThe reason will be displayed to describe this comment to others. Learn more. remove this |
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--set_option pp.all true | ||
--set_option trace.simp_lemmas_cache true | ||
instance : field complex := | ||
{ --field . | ||
add := (+), -- I need this! | ||
zero := 0, -- later crunch proofs won't work without these. | ||
-- neg := complex.neg, | ||
-- zero_add := by crunch, | ||
-- add_zero := by crunch, | ||
-- add_comm := by crunch, | ||
-- add_assoc := by crunch, | ||
-- add_left_neg := by crunch, | ||
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one := 1, | ||
mul := has_mul.mul, | ||
inv := has_inv.inv, | ||
mul_one := by crunch, | ||
one_mul := by crunch, | ||
mul_comm := by crunch3, | ||
mul_assoc := by crunch, | ||
left_distrib := by begin | ||
intros, | ||
apply eq_of_re_eq_and_im_eq, | ||
{ rw [mul_re,add_re,add_re,add_im,mul_re,mul_re], | ||
simp [add_mul,mul_add] }, | ||
{ rw [mul_im,add_re,add_im,add_im,mul_im,mul_im], | ||
simp [add_mul,mul_add] }, | ||
end, | ||
-- left_distrib := by crunch, | ||
right_distrib := by begin | ||
There was a problem hiding this comment. Choose a reason for hiding this commentThe reason will be displayed to describe this comment to others. Learn more.
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intros, | ||
apply eq_of_re_eq_and_im_eq, | ||
{ rw [mul_re,add_re,add_re,add_im,mul_re,mul_re], | ||
simp [add_mul,mul_add] }, | ||
{ rw [mul_im,add_re,add_im,add_im,mul_im,mul_im], | ||
simp [add_mul,mul_add] }, | ||
end, | ||
zero_ne_one := begin | ||
intro H, | ||
suffices : ((0:real):complex).re = ((1:real):complex).re, | ||
{ revert this, | ||
apply zero_ne_one }, | ||
{ show (0:complex).re = (1:complex).re, | ||
rw [←H],refl}, | ||
end, | ||
mul_inv_cancel := begin | ||
intros z H, | ||
have H2 : norm_squared z ≠ 0 := ne_of_gt (norm_squared_pos_of_nonzero z H), | ||
apply eq_of_re_eq_and_im_eq, | ||
{ unfold has_inv.inv complex.inv, | ||
rw [mul_re], | ||
show z.re * (z.re / norm_squared z) - | ||
z.im * (-z.im / norm_squared z) = | ||
(1:complex).re, | ||
suffices : z.re*(z.re/norm_squared z) + -z.im*(-z.im/norm_squared z) = 1, | ||
by simpa, | ||
rw [←mul_div_assoc,←mul_div_assoc,neg_mul_neg,div_add_div_same], | ||
unfold norm_squared at *, | ||
exact div_self H2}, | ||
{ suffices : z.im * (z.re / norm_squared z) + z.re * (-z.im / norm_squared z) = 0, | ||
by simpa, | ||
rw [←mul_div_assoc,←mul_div_assoc,div_add_div_same], | ||
simp [zero_div,mul_comm], | ||
} | ||
end, | ||
inv_mul_cancel := begin | ||
intros z H, | ||
have H2 : norm_squared z ≠ 0 := ne_of_gt (norm_squared_pos_of_nonzero z H), | ||
apply eq_of_re_eq_and_im_eq, | ||
{ unfold has_inv.inv complex.inv, | ||
rw [mul_re], | ||
show (z.re / norm_squared z) * z.re - | ||
(-z.im / norm_squared z) * z.im = | ||
(1:complex).re, | ||
suffices : z.re*(z.re/norm_squared z) + -z.im*(-z.im/norm_squared z) = 1, | ||
by simpa [mul_comm], | ||
rw [←mul_div_assoc,←mul_div_assoc,neg_mul_neg,div_add_div_same], | ||
unfold norm_squared at *, | ||
exact div_self H2 }, | ||
unfold has_inv.inv complex.inv, | ||
rw [mul_im], | ||
show (z.re / norm_squared z) * z.im + | ||
(-z.im / norm_squared z) * z.re = | ||
(1:complex).im, | ||
suffices : z.im * (z.re / norm_squared z) + z.re * (-z.im / norm_squared z) = 0, | ||
by simpa [mul_comm], | ||
rw [←mul_div_assoc,←mul_div_assoc,div_add_div_same], | ||
simp [zero_div,mul_comm] | ||
end, | ||
/- | ||
inv_zero := begin | ||
unfold has_inv.inv complex.inv add_comm_group.zero, | ||
apply eq_of_re_eq_and_im_eq, | ||
split;simp [zero_div], | ||
end, | ||
-/ | ||
-- has_decidable_eq := by apply_instance, | ||
..complex.add_comm_group, | ||
} | ||
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theorem im_eq_zero_of_complex_nat (n : ℕ) : (n:complex).im = 0 := | ||
begin | ||
induction n with d Hd, | ||
There was a problem hiding this comment. Choose a reason for hiding this commentThe reason will be displayed to describe this comment to others. Learn more. indention There was a problem hiding this comment. Choose a reason for hiding this commentThe reason will be displayed to describe this comment to others. Learn more. Are there automatic linters for Lean? In python all those comments would be handled by a single run of There was a problem hiding this comment. Choose a reason for hiding this commentThe reason will be displayed to describe this comment to others. Learn more. Sorry, I thought I'd caught them all. In the past I never indented after a begin. I had a system with no initial indent and no {}s which I have used extensively for months now and it's only through talking about this PR that I have come to understand that it is not quite the right system. In general I don't see the point of this indentation after a begin -- it just means that all of my proofs are indented 2 spaces. There was a problem hiding this comment. Choose a reason for hiding this commentThe reason will be displayed to describe this comment to others. Learn more. If you write tactic proofs all your proofs are indented 2 spaces. It would be different for proof terms, where you have: let x := ... in
have ..,
from _,
have .., by ...,
show ..., ... |
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{ simp }, | ||
{ simp [Hd] }, | ||
end | ||
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theorem of_real_nat_eq_complex_nat {n : ℕ} : ↑(n:ℝ) = (n:complex) := | ||
begin | ||
induction n with d Hd,refl, | ||
show ↑↑(d+1) = ↑(d+1), | ||
rw [nat.cast_add,nat.cast_add,←Hd], | ||
simp | ||
end | ||
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instance char_zero_complex : char_zero complex := | ||
⟨begin | ||
intros, | ||
split, | ||
{ rw [←complex.of_real_nat_eq_complex_nat,←complex.of_real_nat_eq_complex_nat], | ||
intro H, | ||
have real_eq := of_real_injective H, | ||
revert real_eq, | ||
have H2 : char_zero ℝ := by apply_instance, | ||
exact (char_zero.cast_inj ℝ).1 }, | ||
intro H,rw [H], | ||
end⟩ | ||
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theorem of_real_int_eq_complex_int {n : ℤ} : of_real (n:ℝ) = (n:complex) := | ||
begin | ||
cases n with nnat nneg,exact of_real_nat_eq_complex_nat, | ||
rw [int.cast_neg_succ_of_nat,int.cast_neg_succ_of_nat], | ||
rw [←nat.cast_one,←nat.cast_add], | ||
rw [←@nat.cast_one complex,←nat.cast_add], | ||
rw [of_real_eq_coe,←of_real_neg], | ||
rw [of_real_nat_eq_complex_nat], | ||
end | ||
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/- | ||
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I could never get this one working. | ||
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theorem of_real_rat_eq_complex_rat {q : ℚ} : of_real (q:ℝ) = (q:complex) := | ||
begin | ||
rw [rat.num_denom q], -- this line doesn't even work now. I don't even understand goal. | ||
rw [rat.cast_mk q.num ↑(q.denom)], | ||
rw [rat.cast_mk q.num ↑(q.denom)], | ||
rw [div_eq_mul_inv,div_eq_mul_inv,←of_real_mul], | ||
rw [of_real_int_eq_complex_int], | ||
rw [←@of_real_int_eq_complex_int ((q.denom):ℤ)], | ||
rw [of_real_inv], | ||
tactic.swap, | ||
apply_instance, | ||
-- exact complex.char_zero_complex, -- times out | ||
admit, | ||
end | ||
-/ | ||
-- TODO : instance : topological_ring complex := missing | ||
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end complex |
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I don't think you need this method. Just replace
by crunch
withby simp [add_mul, mul_add, eq_iff_re_eq_and_im_eq] {contextual:=tt}