This file contains some miscellaneous prerequisites for proving the Marcinkiewisz real interpolation theorem. There are the following sections:
- Interface for using cutoff functions
- Results about the particular choice of scale
- Some tools for measure theory computations
- Results about truncations of a function
- Measurability properties of truncations
- Truncations and Lp spaces
Interface for using cutoff functions #
A ToneCouple is a couple of two monotone functions that are practically inverses of each
other. It is used in the proof of the real interpolation theorem.
Note: originally it seemed useful to make the possible choice of this function general in the proof of the real inteprolation theorem. However, in the end really only one function works for all the different cases. This infrastructure, however, could potentially still be useful, if one would like to try to improve the constant.
- mon : Bool
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A StrictRangeToneCouple is a ToneCouple for which the functions in the couple, when
restricted to Ioo 0 ∞, map to Ioo 0 ∞.
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A scaled power function gives rise to a ToneCouple.
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- One or more equations did not get rendered due to their size.
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Results about the particular choice of scale #
The proof of the real interpolation theorem will estimate
`distribution (trunc f A(t)) t` and `distribution (truncCompl f A(t)) t` for a
function `A`. The function `A` can be given a closed-form expression that works for
_all_ cases in the real interpolation theorem, because of the computation rules available
for elements in `ℝ≥0∞` that avoid the need for a limiting procedure, e.g. `⊤⁻¹ = 0`.
The function `A` will be of the form `A(t) = (t / d) ^ σ` for particular choices of `d` and
`σ`. This section contatins results about the scale `d`.
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- One or more equations did not get rendered due to their size.
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The particular choice of scaled power function that works in the proof of the real interpolation theorem.
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- ChoiceScale.spf_ch ht hq₀q₁ hp₀ hq₀ hp₁ hq₁ hp₀p₁ hC₀ hC₁ hF = { σ := ComputationsChoiceExponent.ζ p₀ q₀ p₁ q₁ t, d := ChoiceScale.d, hd := ⋯, hd' := ⋯, hσ := ⋯ }
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Some tools for measure theory computations #
A collection of small lemmas to help with integral manipulations.
Results about truncations of a function #
The t-truncation of a function f.
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The complement of a t-truncation of a function f.
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A function to deal with truncations and complement of truncations in one go.
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A function is the complement if its truncation and the complement of the truncation.
Alias of MeasureTheory.trunc_add_truncCompl.
A function is the complement if its truncation and the complement of the truncation.
If the truncation parameter is non-positive, the truncation vanishes.
If the truncation parameter is non-positive, the complement of the truncation is the function itself.
Measurability properties of truncations #
A small lemma that is helpful for rewriting
The norm of the truncation is monotone in the truncation parameter
The norm of the complement of the truncation is antitone in the truncation parameter
The norm of the truncation is meaurable in the truncation parameter
The norm of the complement of the truncation is measurable in the truncation parameter
Truncations and L-p spaces #
If f is in Lp, the truncation is element of Lq for q ≥ p.
If f is in Lp, the complement of the truncation is in Lq for q ≤ p.
Some results about the integrals of truncations #
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Extract expressions for the lower Lebesgue integral of power functions