The Mellin transform

We define the Mellin transform of a locally integrable function on Ioi 0, and show it is differentiable in a suitable vertical strip.

Main statements

• mellin : the Mellin transform ∫ (t : ℝ) in Ioi 0, t ^ (s - 1) • f t, where s is a complex number.
• HasMellin: shorthand asserting that the Mellin transform exists and has a given value (analogous to HasSum).
• mellin_differentiableAt_of_isBigO_rpow : if f is O(x ^ (-a)) at infinity, and O(x ^ (-b)) at 0, then mellin f is holomorphic on the domain b < re s < a.

theorem Complex.cpow_mul_ofReal_nonneg {x : ℝ} (hx : 0 ≤ x) (y : ℝ) (z : ℂ) : ↑x ^ (↑y * z) = ↑(x ^ y) ^ z

def MellinConvergent {E : Type u_1} [NormedAddCommGroup E] [NormedSpace ℂ E] (f : ℝ → E) (s : ℂ) : Prop

Predicate on f and s asserting that the Mellin integral is well-defined.

Equations

• MellinConvergent f s = MeasureTheory.IntegrableOn (fun (t : ℝ) => ↑t ^ (s - 1) • f t) (Set.Ioi 0)

theorem MellinConvergent.const_smul {E : Type u_1} [NormedAddCommGroup E] [NormedSpace ℂ E] {f : ℝ → E} {s : ℂ} (hf : MellinConvergent f s) {𝕜 : Type u_2} [NontriviallyNormedField 𝕜] [NormedSpace 𝕜 E] [SMulCommClass ℂ 𝕜 E] (c : 𝕜) : MellinConvergent (fun (t : ℝ) => c • f t) s

theorem MellinConvergent.cpow_smul {E : Type u_1} [NormedAddCommGroup E] [NormedSpace ℂ E] {f : ℝ → E} {s : ℂ} {a : ℂ} : MellinConvergent (fun (t : ℝ) => ↑t ^ a • f t) s ↔ MellinConvergent f (s + a)

theorem MellinConvergent.div_const {f : ℝ → ℂ} {s : ℂ} (hf : MellinConvergent f s) (a : ℂ) : MellinConvergent (fun (t : ℝ) => f t / a) s

theorem MellinConvergent.comp_mul_left {E : Type u_1} [NormedAddCommGroup E] [NormedSpace ℂ E] {f : ℝ → E} {s : ℂ} {a : ℝ} (ha : 0 < a) : MellinConvergent (fun (t : ℝ) => f (a * t)) s ↔ MellinConvergent f s

theorem MellinConvergent.comp_rpow {E : Type u_1} [NormedAddCommGroup E] [NormedSpace ℂ E] {f : ℝ → E} {s : ℂ} {a : ℝ} (ha : a ≠ 0) : MellinConvergent (fun (t : ℝ) => f (t ^ a)) s ↔ MellinConvergent f (s / ↑a)

def mellin {E : Type u_1} [NormedAddCommGroup E] [NormedSpace ℂ E] (f : ℝ → E) (s : ℂ) : E

The Mellin transform of a function f (for a complex exponent s), defined as the integral of t ^ (s - 1) • f over Ioi 0.

Equations

• mellin f s = ∫ (t : ℝ) in Set.Ioi 0, ↑t ^ (s - 1) • f t

theorem mellin_cpow_smul {E : Type u_1} [NormedAddCommGroup E] [NormedSpace ℂ E] (f : ℝ → E) (s : ℂ) (a : ℂ) : mellin (fun (t : ℝ) => ↑t ^ a • f t) s = mellin f (s + a)

theorem mellin_const_smul {E : Type u_1} [NormedAddCommGroup E] [NormedSpace ℂ E] (f : ℝ → E) (s : ℂ) {𝕜 : Type u_2} [NontriviallyNormedField 𝕜] [NormedSpace 𝕜 E] [SMulCommClass ℂ 𝕜 E] (c : 𝕜) : mellin (fun (t : ℝ) => c • f t) s = c • mellin f s

theorem mellin_div_const (f : ℝ → ℂ) (s : ℂ) (a : ℂ) : mellin (fun (t : ℝ) => f t / a) s = mellin f s / a

theorem mellin_comp_rpow {E : Type u_1} [NormedAddCommGroup E] [NormedSpace ℂ E] (f : ℝ → E) (s : ℂ) (a : ℝ) : mellin (fun (t : ℝ) => f (t ^ a)) s = |a|⁻¹ • mellin f (s / ↑a)

theorem mellin_comp_mul_left {E : Type u_1} [NormedAddCommGroup E] [NormedSpace ℂ E] (f : ℝ → E) (s : ℂ) {a : ℝ} (ha : 0 < a) : mellin (fun (t : ℝ) => f (a * t)) s = ↑a ^ (-s) • mellin f s

theorem mellin_comp_mul_right {E : Type u_1} [NormedAddCommGroup E] [NormedSpace ℂ E] (f : ℝ → E) (s : ℂ) {a : ℝ} (ha : 0 < a) : mellin (fun (t : ℝ) => f (t * a)) s = ↑a ^ (-s) • mellin f s

theorem mellin_comp_inv {E : Type u_1} [NormedAddCommGroup E] [NormedSpace ℂ E] (f : ℝ → E) (s : ℂ) : mellin (fun (t : ℝ) => f t⁻¹) s = mellin f (-s)

def HasMellin {E : Type u_1} [NormedAddCommGroup E] [NormedSpace ℂ E] (f : ℝ → E) (s : ℂ) (m : E) : Prop

Predicate standing for "the Mellin transform of f is defined at s and equal to m". This shortens some arguments.

Equations

• HasMellin f s m = (MellinConvergent f s ∧ mellin f s = m)

theorem hasMellin_add {E : Type u_1} [NormedAddCommGroup E] [NormedSpace ℂ E] {f : ℝ → E} {g : ℝ → E} {s : ℂ} (hf : MellinConvergent f s) (hg : MellinConvergent g s) : HasMellin (fun (t : ℝ) => f t + g t) s (mellin f s + mellin g s)

theorem hasMellin_sub {E : Type u_1} [NormedAddCommGroup E] [NormedSpace ℂ E] {f : ℝ → E} {g : ℝ → E} {s : ℂ} (hf : MellinConvergent f s) (hg : MellinConvergent g s) : HasMellin (fun (t : ℝ) => f t - g t) s (mellin f s - mellin g s)

Convergence of Mellin transform integrals

theorem mellin_convergent_iff_norm {E : Type u_1} [NormedAddCommGroup E] [NormedSpace ℂ E] {f : ℝ → E} {T : Set ℝ} (hT : T ⊆ Set.Ioi 0) (hT' : MeasurableSet T) (hfc : MeasureTheory.AEStronglyMeasurable f (MeasureTheory.Measure.restrict MeasureTheory.volume (Set.Ioi 0))) {s : ℂ} : MeasureTheory.IntegrableOn (fun (t : ℝ) => ↑t ^ (s - 1) • f t) T ↔ MeasureTheory.IntegrableOn (fun (t : ℝ) => t ^ (s.re - 1) * ‖f t‖) T

Auxiliary lemma to reduce convergence statements from vector-valued functions to real scalar-valued functions.

theorem mellin_convergent_top_of_isBigO {f : ℝ → ℝ} (hfc : MeasureTheory.AEStronglyMeasurable f (MeasureTheory.Measure.restrict MeasureTheory.volume (Set.Ioi 0))) {a : ℝ} {s : ℝ} (hf : f =O[Filter.atTop] fun (x : ℝ) => x ^ (-a)) (hs : s < a) : ∃ (c : ℝ), 0 < c ∧ MeasureTheory.IntegrableOn (fun (t : ℝ) => t ^ (s - 1) * f t) (Set.Ioi c)

If f is a locally integrable real-valued function which is O(x ^ (-a)) at ∞, then for any s < a, its Mellin transform converges on some neighbourhood of +∞.

theorem mellin_convergent_zero_of_isBigO {b : ℝ} {f : ℝ → ℝ} (hfc : MeasureTheory.AEStronglyMeasurable f (MeasureTheory.Measure.restrict MeasureTheory.volume (Set.Ioi 0))) (hf : f =O[nhdsWithin 0 (Set.Ioi 0)] fun (x : ℝ) => x ^ (-b)) {s : ℝ} (hs : b < s) : ∃ (c : ℝ), 0 < c ∧ MeasureTheory.IntegrableOn (fun (t : ℝ) => t ^ (s - 1) * f t) (Set.Ioc 0 c)

If f is a locally integrable real-valued function which is O(x ^ (-b)) at 0, then for any b < s, its Mellin transform converges on some right neighbourhood of 0.

theorem mellin_convergent_of_isBigO_scalar {a : ℝ} {b : ℝ} {f : ℝ → ℝ} {s : ℝ} (hfc : MeasureTheory.LocallyIntegrableOn f (Set.Ioi 0)) (hf_top : f =O[Filter.atTop] fun (x : ℝ) => x ^ (-a)) (hs_top : s < a) (hf_bot : f =O[nhdsWithin 0 (Set.Ioi 0)] fun (x : ℝ) => x ^ (-b)) (hs_bot : b < s) : MeasureTheory.IntegrableOn (fun (t : ℝ) => t ^ (s - 1) * f t) (Set.Ioi 0)

If f is a locally integrable real-valued function on Ioi 0 which is O(x ^ (-a)) at ∞ and O(x ^ (-b)) at 0, then its Mellin transform integral converges for b < s < a.

theorem mellinConvergent_of_isBigO_rpow {E : Type u_1} [NormedAddCommGroup E] [NormedSpace ℂ E] {a : ℝ} {b : ℝ} {f : ℝ → E} {s : ℂ} (hfc : MeasureTheory.LocallyIntegrableOn f (Set.Ioi 0)) (hf_top : f =O[Filter.atTop] fun (x : ℝ) => x ^ (-a)) (hs_top : s.re < a) (hf_bot : f =O[nhdsWithin 0 (Set.Ioi 0)] fun (x : ℝ) => x ^ (-b)) (hs_bot : b < s.re) : MellinConvergent f s

theorem isBigO_rpow_top_log_smul {E : Type u_1} [NormedAddCommGroup E] [NormedSpace ℝ E] {a : ℝ} {b : ℝ} {f : ℝ → E} (hab : b < a) (hf : f =O[Filter.atTop] fun (x : ℝ) => x ^ (-a)) : (fun (t : ℝ) => Real.log t • f t) =O[Filter.atTop] fun (x : ℝ) => x ^ (-b)

If f is O(x ^ (-a)) as x → +∞, then log • f is O(x ^ (-b)) for every b < a.

theorem isBigO_rpow_zero_log_smul {E : Type u_1} [NormedAddCommGroup E] [NormedSpace ℝ E] {a : ℝ} {b : ℝ} {f : ℝ → E} (hab : a < b) (hf : f =O[nhdsWithin 0 (Set.Ioi 0)] fun (x : ℝ) => x ^ (-a)) : (fun (t : ℝ) => Real.log t • f t) =O[nhdsWithin 0 (Set.Ioi 0)] fun (x : ℝ) => x ^ (-b)

If f is O(x ^ (-a)) as x → 0, then log • f is O(x ^ (-b)) for every a < b.

theorem mellin_hasDerivAt_of_isBigO_rpow {E : Type u_1} [NormedAddCommGroup E] [CompleteSpace E] [NormedSpace ℂ E] {a : ℝ} {b : ℝ} {f : ℝ → E} {s : ℂ} (hfc : MeasureTheory.LocallyIntegrableOn f (Set.Ioi 0)) (hf_top : f =O[Filter.atTop] fun (x : ℝ) => x ^ (-a)) (hs_top : s.re < a) (hf_bot : f =O[nhdsWithin 0 (Set.Ioi 0)] fun (x : ℝ) => x ^ (-b)) (hs_bot : b < s.re) : MellinConvergent (fun (t : ℝ) => Real.log t • f t) s ∧ HasDerivAt (mellin f) (mellin (fun (t : ℝ) => Real.log t • f t) s) s

Suppose f is locally integrable on (0, ∞), is O(x ^ (-a)) as x → ∞, and is O(x ^ (-b)) as x → 0. Then its Mellin transform is differentiable on the domain b < re s < a, with derivative equal to the Mellin transform of log • f.

theorem mellin_differentiableAt_of_isBigO_rpow {E : Type u_1} [NormedAddCommGroup E] [CompleteSpace E] [NormedSpace ℂ E] {a : ℝ} {b : ℝ} {f : ℝ → E} {s : ℂ} (hfc : MeasureTheory.LocallyIntegrableOn f (Set.Ioi 0)) (hf_top : f =O[Filter.atTop] fun (x : ℝ) => x ^ (-a)) (hs_top : s.re < a) (hf_bot : f =O[nhdsWithin 0 (Set.Ioi 0)] fun (x : ℝ) => x ^ (-b)) (hs_bot : b < s.re) : DifferentiableAt ℂ (mellin f) s

Suppose f is locally integrable on (0, ∞), is O(x ^ (-a)) as x → ∞, and is O(x ^ (-b)) as x → 0. Then its Mellin transform is differentiable on the domain b < re s < a.

theorem mellinConvergent_of_isBigO_rpow_exp {E : Type u_1} [NormedAddCommGroup E] [NormedSpace ℂ E] {a : ℝ} {b : ℝ} (ha : 0 < a) {f : ℝ → E} {s : ℂ} (hfc : MeasureTheory.LocallyIntegrableOn f (Set.Ioi 0)) (hf_top : f =O[Filter.atTop] fun (t : ℝ) => Real.exp (-a * t)) (hf_bot : f =O[nhdsWithin 0 (Set.Ioi 0)] fun (x : ℝ) => x ^ (-b)) (hs_bot : b < s.re) : MellinConvergent f s

If f is locally integrable, decays exponentially at infinity, and is O(x ^ (-b)) at 0, then its Mellin transform converges for b < s.re.

theorem mellin_differentiableAt_of_isBigO_rpow_exp {E : Type u_1} [NormedAddCommGroup E] [CompleteSpace E] [NormedSpace ℂ E] {a : ℝ} {b : ℝ} (ha : 0 < a) {f : ℝ → E} {s : ℂ} (hfc : MeasureTheory.LocallyIntegrableOn f (Set.Ioi 0)) (hf_top : f =O[Filter.atTop] fun (t : ℝ) => Real.exp (-a * t)) (hf_bot : f =O[nhdsWithin 0 (Set.Ioi 0)] fun (x : ℝ) => x ^ (-b)) (hs_bot : b < s.re) : DifferentiableAt ℂ (mellin f) s

If f is locally integrable, decays exponentially at infinity, and is O(x ^ (-b)) at 0, then its Mellin transform is holomorphic on b < s.re.

Mellin transforms of functions on Ioc 0 1

theorem hasMellin_one_Ioc {s : ℂ} (hs : 0 < s.re) : HasMellin (Set.indicator (Set.Ioc 0 1) fun (x : ℝ) => 1) s (1 / s)

The Mellin transform of the indicator function of Ioc 0 1.

theorem hasMellin_cpow_Ioc (a : ℂ) {s : ℂ} (hs : 0 < s.re + a.re) : HasMellin (Set.indicator (Set.Ioc 0 1) fun (t : ℝ) => ↑t ^ a) s (1 / (s + a))

The Mellin transform of a power function restricted to Ioc 0 1.