The complex numbers

The complex numbers are modelled as ℝ^2 in the obvious way and it is shown that they form a field of characteristic zero. The result that the complex numbers are algebraically closed, see FieldTheory.AlgebraicClosure.

Definition and basic arithmetic

structure Complex : Type

Complex numbers consist of two Reals: a real part re and an imaginary part im.

• re : ℝ

  The real part of a complex number.

• im : ℝ

  The imaginary part of a complex number.

def termℂ : Lean.ParserDescr

Complex numbers consist of two Reals: a real part re and an imaginary part im.

Equations

• termℂ = Lean.ParserDescr.node `termℂ 1024 (Lean.ParserDescr.symbol "ℂ")

noncomputable instance Complex.instDecidableEqComplex : DecidableEq ℂ

Equations

• Complex.instDecidableEqComplex = Classical.decEq ℂ

@[simp]

theorem Complex.equivRealProd_apply (z : ℂ) : Complex.equivRealProd z = (z.re, z.im)

def Complex.equivRealProd : ℂ ≃ ℝ × ℝ

The equivalence between the complex numbers and ℝ × ℝ.

Equations

• One or more equations did not get rendered due to their size.

@[simp]

theorem Complex.eta (z : ℂ) : { re := z.re, im := z.im } = z

theorem Complex.ext {z : ℂ} {w : ℂ} : z.re = w.re → z.im = w.im → z = w

theorem Complex.ext_iff {z : ℂ} {w : ℂ} : z = w ↔ z.re = w.re ∧ z.im = w.im

theorem Complex.re_surjective : Function.Surjective Complex.re

theorem Complex.im_surjective : Function.Surjective Complex.im

@[simp]

theorem Complex.range_re : Set.range Complex.re = Set.univ

@[simp]

theorem Complex.range_im : Set.range Complex.im = Set.univ

def Complex.ofReal' (r : ℝ) : ℂ

The natural inclusion of the real numbers into the complex numbers. The name Complex.ofReal is reserved for the bundled homomorphism.

Equations

• ↑r = { re := r, im := 0 }

instance Complex.instCoeRealComplex : Coe ℝ ℂ

Equations

• Complex.instCoeRealComplex = { coe := Complex.ofReal' }

@[simp]

theorem Complex.ofReal_re (r : ℝ) : (↑r).re = r

@[simp]

theorem Complex.ofReal_im (r : ℝ) : (↑r).im = 0

theorem Complex.ofReal_def (r : ℝ) : ↑r = { re := r, im := 0 }

@[simp]

theorem Complex.ofReal_inj {z : ℝ} {w : ℝ} : ↑z = ↑w ↔ z = w

theorem Complex.ofReal_injective : Function.Injective Complex.ofReal'

instance Complex.canLift : CanLift ℂ ℝ Complex.ofReal' fun (z : ℂ) => z.im = 0

Equations

• Complex.canLift = ⋯

def Complex.Set.reProdIm (s : Set ℝ) (t : Set ℝ) : Set ℂ

The product of a set on the real axis and a set on the imaginary axis of the complex plane, denoted by s ×ℂ t.

Equations

• s ×ℂ t = Complex.re ⁻¹' s ∩ Complex.im ⁻¹' t

def Complex.«term_×ℂ_» : Lean.TrailingParserDescr

Equations

• Complex.«term_×ℂ_» = Lean.ParserDescr.trailingNode `Complex.term_×ℂ_ 72 72 (Lean.ParserDescr.binary `andthen (Lean.ParserDescr.symbol " ×ℂ ") (Lean.ParserDescr.cat `term 73))

theorem Complex.mem_reProdIm {z : ℂ} {s : Set ℝ} {t : Set ℝ} : z ∈ s ×ℂ t ↔ z.re ∈ s ∧ z.im ∈ t

instance Complex.instZeroComplex : Zero ℂ

Equations

• Complex.instZeroComplex = { zero := ↑0 }

instance Complex.instInhabitedComplex : Inhabited ℂ

Equations

• Complex.instInhabitedComplex = { default := 0 }

@[simp]

theorem Complex.zero_re : 0.re = 0

@[simp]

theorem Complex.zero_im : 0.im = 0

@[simp]

theorem Complex.ofReal_zero : ↑0 = 0

@[simp]

theorem Complex.ofReal_eq_zero {z : ℝ} : ↑z = 0 ↔ z = 0

theorem Complex.ofReal_ne_zero {z : ℝ} : ↑z ≠ 0 ↔ z ≠ 0

instance Complex.instOneComplex : One ℂ

Equations

• Complex.instOneComplex = { one := ↑1 }

@[simp]

theorem Complex.one_re : 1.re = 1

@[simp]

theorem Complex.one_im : 1.im = 0

@[simp]

theorem Complex.ofReal_one : ↑1 = 1

@[simp]

theorem Complex.ofReal_eq_one {z : ℝ} : ↑z = 1 ↔ z = 1

theorem Complex.ofReal_ne_one {z : ℝ} : ↑z ≠ 1 ↔ z ≠ 1

instance Complex.instAddComplex : Add ℂ

Equations

• Complex.instAddComplex = { add := fun (z w : ℂ) => { re := z.re + w.re, im := z.im + w.im } }

@[simp]

theorem Complex.add_re (z : ℂ) (w : ℂ) : (z + w).re = z.re + w.re

@[simp]

theorem Complex.add_im (z : ℂ) (w : ℂ) : (z + w).im = z.im + w.im

@[simp]

theorem Complex.ofReal_add (r : ℝ) (s : ℝ) : ↑(r + s) = ↑r + ↑s

instance Complex.instNegComplex : Neg ℂ

Equations

• Complex.instNegComplex = { neg := fun (z : ℂ) => { re := -z.re, im := -z.im } }

@[simp]

theorem Complex.neg_re (z : ℂ) : (-z).re = -z.re

@[simp]

theorem Complex.neg_im (z : ℂ) : (-z).im = -z.im

@[simp]

theorem Complex.ofReal_neg (r : ℝ) : ↑(-r) = -↑r

instance Complex.instSubComplex : Sub ℂ

Equations

• Complex.instSubComplex = { sub := fun (z w : ℂ) => { re := z.re - w.re, im := z.im - w.im } }

instance Complex.instMulComplex : Mul ℂ

Equations

• Complex.instMulComplex = { mul := fun (z w : ℂ) => { re := z.re * w.re - z.im * w.im, im := z.re * w.im + z.im * w.re } }

@[simp]

theorem Complex.mul_re (z : ℂ) (w : ℂ) : (z * w).re = z.re * w.re - z.im * w.im

@[simp]

theorem Complex.mul_im (z : ℂ) (w : ℂ) : (z * w).im = z.re * w.im + z.im * w.re

@[simp]

theorem Complex.ofReal_mul (r : ℝ) (s : ℝ) : ↑(r * s) = ↑r * ↑s

theorem Complex.re_ofReal_mul (r : ℝ) (z : ℂ) : (↑r * z).re = r * z.re

theorem Complex.im_ofReal_mul (r : ℝ) (z : ℂ) : (↑r * z).im = r * z.im

theorem Complex.re_mul_ofReal (z : ℂ) (r : ℝ) : (z * ↑r).re = z.re * r

theorem Complex.im_mul_ofReal (z : ℂ) (r : ℝ) : (z * ↑r).im = z.im * r

theorem Complex.ofReal_mul' (r : ℝ) (z : ℂ) : ↑r * z = { re := r * z.re, im := r * z.im }

The imaginary unit, I

def Complex.I : ℂ

The imaginary unit.

Equations

• Complex.I = { re := 0, im := 1 }

@[simp]

theorem Complex.I_re : Complex.I.re = 0

@[simp]

theorem Complex.I_im : Complex.I.im = 1

@[simp]

theorem Complex.I_mul_I : Complex.I * Complex.I = -1

theorem Complex.I_mul (z : ℂ) : Complex.I * z = { re := -z.im, im := z.re }

@[simp]

theorem Complex.I_ne_zero : Complex.I ≠ 0

theorem Complex.mk_eq_add_mul_I (a : ℝ) (b : ℝ) : { re := a, im := b } = ↑a + ↑b * Complex.I

@[simp]

theorem Complex.re_add_im (z : ℂ) : ↑z.re + ↑z.im * Complex.I = z

theorem Complex.mul_I_re (z : ℂ) : (z * Complex.I).re = -z.im

theorem Complex.mul_I_im (z : ℂ) : (z * Complex.I).im = z.re

theorem Complex.I_mul_re (z : ℂ) : (Complex.I * z).re = -z.im

theorem Complex.I_mul_im (z : ℂ) : (Complex.I * z).im = z.re

@[simp]

theorem Complex.equivRealProd_symm_apply (p : ℝ × ℝ) : Complex.equivRealProd.symm p = ↑p.1 + ↑p.2 * Complex.I

Commutative ring instance and lemmas

instance Complex.instNontrivialComplex : Nontrivial ℂ

Equations

• Complex.instNontrivialComplex = ⋯

instance Complex.instSMulRealComplex {R : Type u_1} [SMul R ℝ] : SMul R ℂ

Equations

• Complex.instSMulRealComplex = { smul := fun (r : R) (x : ℂ) => { re := r • x.re - 0 * x.im, im := r • x.im + 0 * x.re } }

theorem Complex.smul_re {R : Type u_1} [SMul R ℝ] (r : R) (z : ℂ) : (r • z).re = r • z.re

theorem Complex.smul_im {R : Type u_1} [SMul R ℝ] (r : R) (z : ℂ) : (r • z).im = r • z.im

@[simp]

theorem Complex.real_smul {x : ℝ} {z : ℂ} : x • z = ↑x * z

instance Complex.addCommGroup : AddCommGroup ℂ

Equations

• Complex.addCommGroup = AddCommGroup.mk Complex.addCommGroup.proof_11

instance Complex.addGroupWithOne : AddGroupWithOne ℂ

Equations

• One or more equations did not get rendered due to their size.

instance Complex.commRing : CommRing ℂ

Equations

• Complex.commRing = let __src := Complex.addGroupWithOne; CommRing.mk Complex.commRing.proof_13

instance Complex.instRingComplex : Ring ℂ

This shortcut instance ensures we do not find Ring via the noncomputable Complex.field instance.

Equations

• Complex.instRingComplex = inferInstance

instance Complex.instCommSemiringComplex : CommSemiring ℂ

This shortcut instance ensures we do not find CommSemiring via the noncomputable Complex.field instance.

Equations

• Complex.instCommSemiringComplex = inferInstance

instance Complex.instSemiringComplex : Semiring ℂ

This shortcut instance ensures we do not find Semiring via the noncomputable Complex.field instance.

Equations

• Complex.instSemiringComplex = inferInstance

def Complex.reAddGroupHom : ℂ →+ ℝ

The "real part" map, considered as an additive group homomorphism.

Equations

• Complex.reAddGroupHom = { toZeroHom := { toFun := Complex.re, map_zero' := Complex.zero_re }, map_add' := Complex.add_re }

@[simp]

theorem Complex.coe_reAddGroupHom : ⇑Complex.reAddGroupHom = Complex.re

def Complex.imAddGroupHom : ℂ →+ ℝ

The "imaginary part" map, considered as an additive group homomorphism.

Equations

• Complex.imAddGroupHom = { toZeroHom := { toFun := Complex.im, map_zero' := Complex.zero_im }, map_add' := Complex.add_im }

@[simp]

theorem Complex.coe_imAddGroupHom : ⇑Complex.imAddGroupHom = Complex.im

@[simp]

theorem Complex.I_pow_bit0 (n : ℕ) : Complex.I ^ bit0 n = (-1) ^ n

@[simp]

theorem Complex.I_pow_bit1 (n : ℕ) : Complex.I ^ bit1 n = (-1) ^ n * Complex.I

@[simp]

theorem Complex.ofReal_ofNat (n : ℕ) [Nat.AtLeastTwo n] : ↑(OfNat.ofNat n) = OfNat.ofNat n

@[simp]

theorem Complex.re_ofNat (n : ℕ) [Nat.AtLeastTwo n] : (OfNat.ofNat n).re = OfNat.ofNat n

@[simp]

theorem Complex.im_ofNat (n : ℕ) [Nat.AtLeastTwo n] : (OfNat.ofNat n).im = 0

noncomputable instance Complex.instRatCastComplex : RatCast ℂ

Equations

• Complex.instRatCastComplex = { ratCast := fun (q : ℚ) => ↑↑q }

Complex conjugation

instance Complex.instStarRingComplexToNonUnitalNonAssocSemiringToNonUnitalNonAssocCommSemiringToNonUnitalNonAssocCommRingToNonUnitalCommRingCommRing : StarRing ℂ

This defines the complex conjugate as the star operation of the StarRing ℂ. It is recommended to use the ring endomorphism version starRingEnd, available under the notation conj in the locale ComplexConjugate.

Equations

• One or more equations did not get rendered due to their size.

@[simp]

theorem Complex.conj_re (z : ℂ) : ((starRingEnd ℂ) z).re = z.re

@[simp]

theorem Complex.conj_im (z : ℂ) : ((starRingEnd ℂ) z).im = -z.im

theorem Complex.conj_ofReal (r : ℝ) : (starRingEnd ℂ) ↑r = ↑r

@[simp]

theorem Complex.conj_I : (starRingEnd ℂ) Complex.I = -Complex.I

theorem Complex.conj_nat_cast (n : ℕ) : (starRingEnd ℂ) ↑n = ↑n

theorem Complex.conj_ofNat (n : ℕ) [Nat.AtLeastTwo n] : (starRingEnd ℂ) (OfNat.ofNat n) = OfNat.ofNat n

theorem Complex.conj_neg_I : (starRingEnd ℂ) (-Complex.I) = Complex.I

theorem Complex.conj_eq_iff_real {z : ℂ} : (starRingEnd ℂ) z = z ↔ ∃ (r : ℝ), z = ↑r

theorem Complex.conj_eq_iff_re {z : ℂ} : (starRingEnd ℂ) z = z ↔ ↑z.re = z

theorem Complex.conj_eq_iff_im {z : ℂ} : (starRingEnd ℂ) z = z ↔ z.im = 0

@[simp]

theorem Complex.star_def : star = ⇑(starRingEnd ℂ)

Norm squared

def Complex.normSq : ℂ →*₀ ℝ

The norm squared function.

Equations

• One or more equations did not get rendered due to their size.

theorem Complex.normSq_apply (z : ℂ) : Complex.normSq z = z.re * z.re + z.im * z.im

@[simp]

theorem Complex.normSq_ofReal (r : ℝ) : Complex.normSq ↑r = r * r

@[simp]

theorem Complex.normSq_nat_cast (n : ℕ) : Complex.normSq ↑n = ↑n * ↑n

@[simp]

theorem Complex.normSq_int_cast (z : ℤ) : Complex.normSq ↑z = ↑z * ↑z

@[simp]

theorem Complex.normSq_rat_cast (q : ℚ) : Complex.normSq ↑q = ↑q * ↑q

@[simp]

theorem Complex.normSq_ofNat (n : ℕ) [Nat.AtLeastTwo n] : Complex.normSq (OfNat.ofNat n) = OfNat.ofNat n * OfNat.ofNat n

@[simp]

theorem Complex.normSq_mk (x : ℝ) (y : ℝ) : Complex.normSq { re := x, im := y } = x * x + y * y

theorem Complex.normSq_add_mul_I (x : ℝ) (y : ℝ) : Complex.normSq (↑x + ↑y * Complex.I) = x ^ 2 + y ^ 2

theorem Complex.normSq_eq_conj_mul_self {z : ℂ} : ↑(Complex.normSq z) = (starRingEnd ℂ) z * z

theorem Complex.normSq_zero : Complex.normSq 0 = 0

theorem Complex.normSq_one : Complex.normSq 1 = 1

@[simp]

theorem Complex.normSq_I : Complex.normSq Complex.I = 1

theorem Complex.normSq_nonneg (z : ℂ) : 0 ≤ Complex.normSq z

theorem Complex.normSq_eq_zero {z : ℂ} : Complex.normSq z = 0 ↔ z = 0

@[simp]

theorem Complex.normSq_pos {z : ℂ} : 0 < Complex.normSq z ↔ z ≠ 0

@[simp]

theorem Complex.normSq_neg (z : ℂ) : Complex.normSq (-z) = Complex.normSq z

@[simp]

theorem Complex.normSq_conj (z : ℂ) : Complex.normSq ((starRingEnd ℂ) z) = Complex.normSq z

theorem Complex.normSq_mul (z : ℂ) (w : ℂ) : Complex.normSq (z * w) = Complex.normSq z * Complex.normSq w

theorem Complex.normSq_add (z : ℂ) (w : ℂ) : Complex.normSq (z + w) = Complex.normSq z + Complex.normSq w + 2 * (z * (starRingEnd ℂ) w).re

theorem Complex.re_sq_le_normSq (z : ℂ) : z.re * z.re ≤ Complex.normSq z

theorem Complex.im_sq_le_normSq (z : ℂ) : z.im * z.im ≤ Complex.normSq z

theorem Complex.mul_conj (z : ℂ) : z * (starRingEnd ℂ) z = ↑(Complex.normSq z)

theorem Complex.add_conj (z : ℂ) : z + (starRingEnd ℂ) z = ↑(2 * z.re)

def Complex.ofReal : ℝ →+* ℂ

The coercion ℝ → ℂ as a RingHom.

Equations

• One or more equations did not get rendered due to their size.

@[simp]

theorem Complex.ofReal_eq_coe (r : ℝ) : Complex.ofReal r = ↑r

@[simp]

theorem Complex.I_sq : Complex.I ^ 2 = -1

@[simp]

theorem Complex.I_pow_four : Complex.I ^ 4 = 1

@[simp]

theorem Complex.sub_re (z : ℂ) (w : ℂ) : (z - w).re = z.re - w.re

@[simp]

theorem Complex.sub_im (z : ℂ) (w : ℂ) : (z - w).im = z.im - w.im

@[simp]

theorem Complex.ofReal_sub (r : ℝ) (s : ℝ) : ↑(r - s) = ↑r - ↑s

@[simp]

theorem Complex.ofReal_pow (r : ℝ) (n : ℕ) : ↑(r ^ n) = ↑r ^ n

theorem Complex.sub_conj (z : ℂ) : z - (starRingEnd ℂ) z = ↑(2 * z.im) * Complex.I

theorem Complex.normSq_sub (z : ℂ) (w : ℂ) : Complex.normSq (z - w) = Complex.normSq z + Complex.normSq w - 2 * (z * (starRingEnd ℂ) w).re

Inversion

noncomputable instance Complex.instInvComplex : Inv ℂ

Equations

• Complex.instInvComplex = { inv := fun (z : ℂ) => (starRingEnd ℂ) z * ↑(Complex.normSq z)⁻¹ }

theorem Complex.inv_def (z : ℂ) : z⁻¹ = (starRingEnd ℂ) z * ↑(Complex.normSq z)⁻¹

@[simp]

theorem Complex.inv_re (z : ℂ) : z⁻¹.re = z.re / Complex.normSq z

@[simp]

theorem Complex.inv_im (z : ℂ) : z⁻¹.im = -z.im / Complex.normSq z

@[simp]

theorem Complex.ofReal_inv (r : ℝ) : ↑r⁻¹ = (↑r)⁻¹

theorem Complex.inv_zero : 0⁻¹ = 0

theorem Complex.mul_inv_cancel {z : ℂ} (h : z ≠ 0) : z * z⁻¹ = 1

Cast lemmas

@[simp]

theorem Complex.ofReal_nat_cast (n : ℕ) : ↑↑n = ↑n

@[simp]

theorem Complex.nat_cast_re (n : ℕ) : (↑n).re = ↑n

@[simp]

theorem Complex.nat_cast_im (n : ℕ) : (↑n).im = 0

@[simp]

theorem Complex.ofReal_int_cast (n : ℤ) : ↑↑n = ↑n

@[simp]

theorem Complex.int_cast_re (n : ℤ) : (↑n).re = ↑n

@[simp]

theorem Complex.int_cast_im (n : ℤ) : (↑n).im = 0

@[simp]

theorem Complex.ofReal_rat_cast (q : ℚ) : ↑↑q = ↑q

@[simp]

theorem Complex.rat_cast_re (q : ℚ) : (↑q).re = ↑q

@[simp]

theorem Complex.rat_cast_im (q : ℚ) : (↑q).im = 0

theorem Complex.ofReal_nsmul (n : ℕ) (r : ℝ) : ↑(n • r) = n • ↑r

theorem Complex.ofReal_zsmul (n : ℤ) (r : ℝ) : ↑(n • r) = n • ↑r

Field instance and lemmas

noncomputable instance Complex.instField : Field ℂ

Equations

• One or more equations did not get rendered due to their size.

@[simp]

theorem Complex.I_zpow_bit0 (n : ℤ) : Complex.I ^ bit0 n = (-1) ^ n

@[simp]

theorem Complex.I_zpow_bit1 (n : ℤ) : Complex.I ^ bit1 n = (-1) ^ n * Complex.I

theorem Complex.div_re (z : ℂ) (w : ℂ) : (z / w).re = z.re * w.re / Complex.normSq w + z.im * w.im / Complex.normSq w

theorem Complex.div_im (z : ℂ) (w : ℂ) : (z / w).im = z.im * w.re / Complex.normSq w - z.re * w.im / Complex.normSq w

theorem Complex.conj_inv (x : ℂ) : (starRingEnd ℂ) x⁻¹ = ((starRingEnd ℂ) x)⁻¹

@[simp]

theorem Complex.ofReal_div (r : ℝ) (s : ℝ) : ↑(r / s) = ↑r / ↑s

@[simp]

theorem Complex.ofReal_zpow (r : ℝ) (n : ℤ) : ↑(r ^ n) = ↑r ^ n

@[simp]

theorem Complex.div_I (z : ℂ) : z / Complex.I = -(z * Complex.I)

@[simp]

theorem Complex.inv_I : Complex.I⁻¹ = -Complex.I

theorem Complex.normSq_inv (z : ℂ) : Complex.normSq z⁻¹ = (Complex.normSq z)⁻¹

theorem Complex.normSq_div (z : ℂ) (w : ℂ) : Complex.normSq (z / w) = Complex.normSq z / Complex.normSq w

theorem Complex.div_ofReal (z : ℂ) (x : ℝ) : z / ↑x = { re := z.re / x, im := z.im / x }

theorem Complex.div_nat_cast (z : ℂ) (n : ℕ) : z / ↑n = { re := z.re / ↑n, im := z.im / ↑n }

theorem Complex.div_int_cast (z : ℂ) (n : ℤ) : z / ↑n = { re := z.re / ↑n, im := z.im / ↑n }

theorem Complex.div_rat_cast (z : ℂ) (x : ℚ) : z / ↑x = { re := z.re / ↑x, im := z.im / ↑x }

theorem Complex.div_ofNat (z : ℂ) (n : ℕ) [Nat.AtLeastTwo n] : z / OfNat.ofNat n = { re := z.re / OfNat.ofNat n, im := z.im / OfNat.ofNat n }

@[simp]

theorem Complex.div_ofReal_re (z : ℂ) (x : ℝ) : (z / ↑x).re = z.re / x

@[simp]

theorem Complex.div_ofReal_im (z : ℂ) (x : ℝ) : (z / ↑x).im = z.im / x

@[simp]

theorem Complex.div_nat_cast_re (z : ℂ) (n : ℕ) : (z / ↑n).re = z.re / ↑n

@[simp]

theorem Complex.div_nat_cast_im (z : ℂ) (n : ℕ) : (z / ↑n).im = z.im / ↑n

@[simp]

theorem Complex.div_int_cast_re (z : ℂ) (n : ℤ) : (z / ↑n).re = z.re / ↑n

@[simp]

theorem Complex.div_int_cast_im (z : ℂ) (n : ℤ) : (z / ↑n).im = z.im / ↑n

@[simp]

theorem Complex.div_rat_cast_re (z : ℂ) (x : ℚ) : (z / ↑x).re = z.re / ↑x

@[simp]

theorem Complex.div_rat_cast_im (z : ℂ) (x : ℚ) : (z / ↑x).im = z.im / ↑x

@[simp]

theorem Complex.div_ofNat_re (z : ℂ) (n : ℕ) [Nat.AtLeastTwo n] : (z / OfNat.ofNat n).re = z.re / OfNat.ofNat n

@[simp]

theorem Complex.div_ofNat_im (z : ℂ) (n : ℕ) [Nat.AtLeastTwo n] : (z / OfNat.ofNat n).im = z.im / OfNat.ofNat n

Characteristic zero

instance Complex.charZero : CharZero ℂ

Equations

• Complex.charZero = ⋯

theorem Complex.re_eq_add_conj (z : ℂ) : ↑z.re = (z + (starRingEnd ℂ) z) / 2

A complex number z plus its conjugate conj z is 2 times its real part.

theorem Complex.im_eq_sub_conj (z : ℂ) : ↑z.im = (z - (starRingEnd ℂ) z) / (2 * Complex.I)

A complex number z minus its conjugate conj z is 2i times its imaginary part.