Addition of complex numbers

Content created by Louis Wasserman.

Created on 2025-10-11.
Last modified on 2026-01-11.

module complex-numbers.addition-complex-numbers where
Imports 
open import complex-numbers.complex-numbers
open import complex-numbers.conjugation-complex-numbers
open import complex-numbers.similarity-complex-numbers

open import foundation.action-on-identifications-binary-functions
open import foundation.action-on-identifications-functions
open import foundation.dependent-pair-types
open import foundation.identity-types
open import foundation.universe-levels

open import real-numbers.addition-real-numbers
open import real-numbers.dedekind-real-numbers
open import real-numbers.raising-universe-levels-real-numbers

Idea

We introduce addition on the complex numbers by componentwise addition of the real and imaginary parts.

Definition

add-ℂ : {l1 l2 : Level} → ℂ l1 → ℂ l2 → ℂ (l1 ⊔ l2)
add-ℂ (a , b) (c , d) = (a +ℝ c , b +ℝ d)

infixl 35 _+ℂ_
_+ℂ_ : {l1 l2 : Level} → ℂ l1 → ℂ l2 → ℂ (l1 ⊔ l2)
_+ℂ_ = add-ℂ

ap-add-ℂ :
  {l1 l2 : Level} →
  {x x' : ℂ l1} → x ＝ x' → {y y' : ℂ l2} → y ＝ y' →
  x +ℂ y ＝ x' +ℂ y'
ap-add-ℂ = ap-binary add-ℂ

Properties

Addition is commutative

abstract
  commutative-add-ℂ : {l1 l2 : Level} (x : ℂ l1) (y : ℂ l2) → x +ℂ y ＝ y +ℂ x
  commutative-add-ℂ _ _ = eq-ℂ (commutative-add-ℝ _ _) (commutative-add-ℝ _ _)

Addition is associative

abstract
  associative-add-ℂ :
    {l1 l2 l3 : Level} (x : ℂ l1) (y : ℂ l2) (z : ℂ l3) →
    (x +ℂ y) +ℂ z ＝ x +ℂ (y +ℂ z)
  associative-add-ℂ _ _ _ =
    eq-ℂ (associative-add-ℝ _ _ _) (associative-add-ℝ _ _ _)

Unit laws of addition

abstract
  left-unit-law-add-ℂ : {l : Level} (x : ℂ l) → zero-ℂ +ℂ x ＝ x
  left-unit-law-add-ℂ (a , b) =
    eq-ℂ (left-unit-law-add-ℝ a) (left-unit-law-add-ℝ b)

  right-unit-law-add-ℂ : {l : Level} (x : ℂ l) → x +ℂ zero-ℂ ＝ x
  right-unit-law-add-ℂ (a , b) =
    eq-ℂ (right-unit-law-add-ℝ a) (right-unit-law-add-ℝ b)

Inverse laws of addition

abstract
  left-inverse-law-add-ℂ : {l : Level} (x : ℂ l) → sim-ℂ (neg-ℂ x +ℂ x) zero-ℂ
  left-inverse-law-add-ℂ (a , b) =
    ( left-inverse-law-add-ℝ a , left-inverse-law-add-ℝ b)

  right-inverse-law-add-ℂ :
    {l : Level} (x : ℂ l) → sim-ℂ (x +ℂ neg-ℂ x) zero-ℂ
  right-inverse-law-add-ℂ (a , b) =
    ( right-inverse-law-add-ℝ a , right-inverse-law-add-ℝ b)

Similarity is preserved by addition

abstract
  preserves-sim-add-ℂ :
    {l1 l2 l3 l4 : Level} {x : ℂ l1} {x' : ℂ l2} {y : ℂ l3} {y' : ℂ l4} →
    sim-ℂ x x' → sim-ℂ y y' → sim-ℂ (x +ℂ y) (x' +ℂ y')
  preserves-sim-add-ℂ (a~a' , b~b') (c~c' , d~d') =
    ( preserves-sim-add-ℝ a~a' c~c' , preserves-sim-add-ℝ b~b' d~d')

The sum of z and conjugate-ℂ z is double re-ℂ z

abstract
  right-add-conjugate-ℂ :
    {l : Level} (z : ℂ l) → z +ℂ conjugate-ℂ z ＝ complex-ℝ (re-ℂ z +ℝ re-ℂ z)
  right-add-conjugate-ℂ (a +iℂ b) = eq-ℂ refl (eq-right-inverse-law-add-ℝ b)

Swapping laws for addition on complex numbers

module _
  {l1 l2 l3 : Level} (x : ℂ l1) (y : ℂ l2) (z : ℂ l3)
  where

  abstract
    right-swap-add-ℂ :
      (x +ℂ y) +ℂ z ＝ (x +ℂ z) +ℂ y
    right-swap-add-ℂ =
      equational-reasoning
        (x +ℂ y) +ℂ z
        ＝ x +ℂ (y +ℂ z) by associative-add-ℂ x y z
        ＝ x +ℂ (z +ℂ y) by ap (x +ℂ_) (commutative-add-ℂ y z)
        ＝ (x +ℂ z) +ℂ y by inv (associative-add-ℂ x z y)

    left-swap-add-ℂ :
      x +ℂ (y +ℂ z) ＝ y +ℂ (x +ℂ z)
    left-swap-add-ℂ =
      equational-reasoning
        x +ℂ (y +ℂ z)
        ＝ (x +ℂ y) +ℂ z by inv (associative-add-ℂ x y z)
        ＝ (y +ℂ x) +ℂ z by ap (_+ℂ z) (commutative-add-ℂ x y)
        ＝ y +ℂ (x +ℂ z) by associative-add-ℂ y x z

Interchange laws for addition on complex numbers

module _
  {l1 l2 l3 l4 : Level} (x : ℂ l1) (y : ℂ l2) (z : ℂ l3) (w : ℂ l4)
  where

  abstract
    interchange-law-add-add-ℂ : (x +ℂ y) +ℂ (z +ℂ w) ＝ (x +ℂ z) +ℂ (y +ℂ w)
    interchange-law-add-add-ℂ =
      equational-reasoning
        (x +ℂ y) +ℂ (z +ℂ w)
        ＝ x +ℂ (y +ℂ (z +ℂ w)) by associative-add-ℂ _ _ _
        ＝ x +ℂ (z +ℂ (y +ℂ w)) by ap (x +ℂ_) (left-swap-add-ℂ y z w)
        ＝ (x +ℂ z) +ℂ (y +ℂ w) by inv (associative-add-ℂ x z (y +ℂ w))

The inclusion of real numbers preserves addition

abstract
  add-complex-ℝ :
    {l1 l2 : Level} (x : ℝ l1) (y : ℝ l2) →
    complex-ℝ x +ℂ complex-ℝ y ＝ complex-ℝ (x +ℝ y)
  add-complex-ℝ {l1} {l2} x y =
    eq-ℂ
      ( refl)
      ( add-raise-ℝ ∙ ap (raise-ℝ _) (left-unit-law-add-ℝ _))

Recent changes