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Mathlib.Analysis.FunctionalSpaces.BesselPotentialSpace

Bessel potential spaces #

We define the Bessel potential space or Fourier-theoretic Sobolev space, with smoothness parameter s and integrability parameter p. Informally, this space is given as the set of all tempered distributions u such that 𝓕⁻ (1 + ‖ξ‖ ^ 2) ^ (s / 2) 𝓕 u is an Lp function.

Implementation notes #

In Mathlib.Analysis.Distribution.Sobolev the unbundled version TemperedDistribution.MemSobolev is defined as ∃ v : Lp, besselPotential E F s u = v. While it would be possible to define the bundled space in the same way, the existence quantifier makes proving theorems more involved and hence we bundle the Lp function v into the structure.

We also note that since every Lp function uniquely defines a distribution via u = besselPotential E F (-s) v, it would be possible to define the Bessel potential space as a one-field structure of Lp. The approach of having both u and v as part of the structure gives better definitional equalities.

structure BesselPotentialSpace (E : Type u_1) (F : Type u_2) [NormedAddCommGroup E] [NormedAddCommGroup F] [InnerProductSpace E] [FiniteDimensional E] [MeasurableSpace E] [BorelSpace E] [CompleteSpace F] [NormedSpace F] (s : ) (p : ENNReal) [hp : Fact (1 p)] :
Type (max u_1 u_2)

The Bessel potential space of order s : ℝ and p : ℝ≥0∞, also known as the Sobolev space and usually denoted by H^{s,p}.

It is defined as the set of all tempered distributions u such that 𝓕⁻ (1 + ‖x‖ ^ 2) ^ (s / 2) 𝓕 u can be represented by a Lp function v. Both u and v are stored as data to avoid using Classical.choose.

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    The Bessel potential space of order s : ℝ and p : ℝ≥0∞, also known as the Sobolev space and usually denoted by H^{s,p}.

    It is defined as the set of all tempered distributions u such that 𝓕⁻ (1 + ‖x‖ ^ 2) ^ (s / 2) 𝓕 u can be represented by a Lp function v. Both u and v are stored as data to avoid using Classical.choose.

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      The Bessel potential space of order s : ℝ and p : ℝ≥0∞, also known as the Sobolev space and usually denoted by H^{s,p}.

      It is defined as the set of all tempered distributions u such that 𝓕⁻ (1 + ‖x‖ ^ 2) ^ (s / 2) 𝓕 u can be represented by a Lp function v. Both u and v are stored as data to avoid using Classical.choose.

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      • One or more equations did not get rendered due to their size.
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        The Bessel potential space of order s : ℝ and p : ℝ≥0∞, also known as the Sobolev space and usually denoted by H^{s,p}.

        It is defined as the set of all tempered distributions u such that 𝓕⁻ (1 + ‖x‖ ^ 2) ^ (s / 2) 𝓕 u can be represented by a Lp function v. Both u and v are stored as data to avoid using Classical.choose.

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        • One or more equations did not get rendered due to their size.
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          The Bessel potential space of order s : ℝ and p : ℝ≥0∞, also known as the Sobolev space and usually denoted by H^{s,p}.

          It is defined as the set of all tempered distributions u such that 𝓕⁻ (1 + ‖x‖ ^ 2) ^ (s / 2) 𝓕 u can be represented by a Lp function v. Both u and v are stored as data to avoid using Classical.choose.

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          • One or more equations did not get rendered due to their size.
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            theorem BesselPotentialSpace.ext {E : Type u_1} {F : Type u_2} [NormedAddCommGroup E] [NormedAddCommGroup F] [InnerProductSpace E] [FiniteDimensional E] [MeasurableSpace E] [BorelSpace E] [CompleteSpace F] [NormedSpace F] {s : } {p : ENNReal} [hp : Fact (1 p)] {f g : BesselPotentialSpace E F s p} (h₁ : f = g) :
            f = g
            def BesselPotentialSpace.copy {E : Type u_1} {F : Type u_2} [NormedAddCommGroup E] [NormedAddCommGroup F] [InnerProductSpace E] [FiniteDimensional E] [MeasurableSpace E] [BorelSpace E] [CompleteSpace F] [NormedSpace F] {s : } {p : ENNReal} [hp : Fact (1 p)] (s' : ) (f : BesselPotentialSpace E F s p) (hs : s = s' := by grind) :

            Transfer a Sobolev function in H^{s,p} to H^{s', p} given that s = s'.

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              @[simp]
              theorem BesselPotentialSpace.toDistr_copy {E : Type u_1} {F : Type u_2} [NormedAddCommGroup E] [NormedAddCommGroup F] [InnerProductSpace E] [FiniteDimensional E] [MeasurableSpace E] [BorelSpace E] [CompleteSpace F] [NormedSpace F] {s s' : } {p : ENNReal} [hp : Fact (1 p)] (f : BesselPotentialSpace E F s p) (hs : s = s') :
              (copy s' f ) = f
              @[simp]
              theorem BesselPotentialSpace.toLp_copy {E : Type u_1} {F : Type u_2} [NormedAddCommGroup E] [NormedAddCommGroup F] [InnerProductSpace E] [FiniteDimensional E] [MeasurableSpace E] [BorelSpace E] [CompleteSpace F] [NormedSpace F] {s s' : } {p : ENNReal} [hp : Fact (1 p)] (f : BesselPotentialSpace E F s p) (hs : s = s') :
              (copy s' f ).toLp = f.toLp
              @[instance_reducible]
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              @[instance_reducible]
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              @[simp]
              theorem BesselPotentialSpace.toDistr_add {E : Type u_1} {F : Type u_2} [NormedAddCommGroup E] [NormedAddCommGroup F] [InnerProductSpace E] [FiniteDimensional E] [MeasurableSpace E] [BorelSpace E] [CompleteSpace F] [NormedSpace F] {s : } {p : ENNReal} [hp : Fact (1 p)] (f g : BesselPotentialSpace E F s p) :
              ↑(f + g) = f + g
              @[instance_reducible]
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              @[simp]
              theorem BesselPotentialSpace.toDistr_sub {E : Type u_1} {F : Type u_2} [NormedAddCommGroup E] [NormedAddCommGroup F] [InnerProductSpace E] [FiniteDimensional E] [MeasurableSpace E] [BorelSpace E] [CompleteSpace F] [NormedSpace F] {s : } {p : ENNReal} [hp : Fact (1 p)] (f g : BesselPotentialSpace E F s p) :
              ↑(f - g) = f - g
              @[instance_reducible]
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              @[instance_reducible]
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              Coercion to tempered distributions as an additive homomorphism.

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                @[instance_reducible]
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                The map u ↦ 𝓕⁻ (1 + ‖x‖ ^ 2) ^ (s / 2) 𝓕 u as a linear map from H^{s,p} to Lp.

                See toLpₗᵢ for the linear isometry equivalence.

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                  Every Lp function defines a Sobolev function by f ↦ besselPotential E F (-s) f.

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                    @[instance_reducible]
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                    The linear isometry between H^{s,p} and Lp.

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                      @[instance_reducible]
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                      Every unbundled Sobolev tempered distribution defines an element in H^{s, p}.

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