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MathlibNt.AnalyticNumberTheory.Chen1973.Chen1973Lemma5SwitchedTripleSource

Chen 1973, Lemma 5: the actual switched triple source #

This file freezes the opening of Lemma 5 on pp. 116--119 of Chen's original scan. It keeps the actual von Mangoldt coefficient, Chen's finite Perron kernel Φ(x/(p₁p₂n)), and the reciprocal logarithmic weight. In particular it does not replace the source by an arbitrary coefficient sequence.

The analytic estimates in Lemmas 5--6 are not asserted here. The first layer is the exact finite bookkeeping used before those estimates: the prime-triple carrier, the Selberg-square expansion, the principal/nonprincipal partition, and the resulting M₁-M₃+M₄ identity.

A natural cutoff version of Chen's Q=∏_{2≤p<x^(1/4)}p. The relation to the real fourth root is carried separately, so the finite product never uses a fake real-indexed finset.

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    An honest natural cutoff realizes the source's strict real fourth-root cutoff when these membership predicates agree.

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      theorem AnalyticNumberTheory.LargeSieve.mem_chen1973Lemma5QCarrier_iff {x z4 p : } (hz : Chen1973FourthRootCutoff x z4) (hp : Nat.Prime p) :
      p {qFinset.range z4 | 2 q Nat.Prime q} 2 p p < x ^ (1 / 4)

      Under the explicit cutoff bridge, membership in the finite Q carrier is exactly the source condition 2 ≤ p < x^(1/4).

      The finite pair carrier printed at the start of Lemma 5: x^(1/10)<p₁≤x^(1/3)<p₂≤(x/p₁)^(1/2).

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        The actual finite triple carrier counted by Ω, before the coprimality sieve is imposed.

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          Chen's Ω: prime triples in the printed carrier for which (x-p₁p₂p₃,Q)=1.

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            Literal membership conditions for the finite Ω carrier.

            Chen p. 115: f(k)=φ(k)∏_{p∣k}(p-2)/(p-1).

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              The normalized finite denominator in Chen's literal Selberg coefficient.

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                The actual coefficient λ_d defined immediately before Lemma 5 (p. 115). R is the honest natural version of x^(1/4-ε/2).

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                  The finite n≤x/(p₁p₂) carrier used after switching p₃ to Λ(n).

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                    The literal source weight Λ(n) Φ(x/(p₁p₂n)) / log(x/(p₁p₂)). The value at a zero denominator is totalized by Lean's field operations; on the prime-pair carrier the later analytic development proves the required positivity separately.

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                      The finite Selberg divisor sum appearing inside the square in (5).

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                        The literal switched Λ·Φ/log mass on the Ω carrier. Keeping this quantity separate avoids silently replacing Chen's smoothed source by a bare cardinality.

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                          theorem AnalyticNumberTheory.LargeSieve.chen1973Lemma5SmoothedWeight_nonneg {x : } (hx : 1 < x) {pp : × } {n : } (hlog : 0 Real.log (x / (pp.1 * pp.2))) :

                          The source weight is nonnegative once its (printed) logarithmic denominator is known to be nonnegative. The nonnegativity of Λ and of Chen's finite Perron kernel are discharged internally.

                          The finite product defining Q is nonzero.

                          theorem AnalyticNumberTheory.LargeSieve.chen1973Lemma5DivisorSum_eq_one_of_coprime {x z4 : } {lambda : } (hlambda : lambda 1 = 1) {pp : × } {n : } (hcop : (x - pp.1 * pp.2 * n).Coprime (chen1973Lemma5Q z4)) :
                          chen1973Lemma5DivisorSum x z4 lambda pp n = 1

                          On an Ω residue, every divisor of Q which divides the residue is one; therefore a Selberg divisor sum normalized by λ₁=1 is exactly one.

                          Exact reindexing of the smoothed Ω source into the switched pair/n coordinates used by the Selberg square.

                          The actual smoothed Ω mass is bounded by the actual Selberg square. The only hypotheses are λ₁=1 and nonnegativity of the source weights; the latter is supplied by chen1973Lemma5SmoothedWeight_nonneg from the genuine Λ·Φ/log definition.

                          A separate, honest cardinal bridge. It applies when the retained smoothed weight is pointwise at least one; this condition is deliberately explicit and is not conflated with mere nonnegativity of the Perron weight.

                          theorem AnalyticNumberTheory.LargeSieve.chen1973Lemma5Omega_le_SelbergSquare {x z4 : } {lambda : } (hlambda : lambda 1 = 1) (hw0 : ppchen1973Lemma5PrimePairs x, nchen1973Lemma5NCarrier x pp, 0 chen1973Lemma5SmoothedWeight x pp n) (hw1 : tchen1973Lemma5OmegaCarrier x z4, 1 chen1973Lemma5SmoothedWeight x t.1 t.2) :

                          Cardinal form of the Ω--Selberg-square bridge, with the genuinely stronger pointwise lower bound isolated from the internally proved sign condition.

                          Expansion of Chen's Selberg square into the two divisor variables.

                          The complementary principal contribution (p₁p₂n,d₁d₂)>1, source M₃.

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                            Pure finite partition of the principal sum into good and bad gcd lanes.

                            noncomputable def AnalyticNumberTheory.LargeSieve.chen1973Lemma5M1 (x z4 : ) (lambda : ) :

                            Source M₁: the unrestricted principal-character main term.

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                              noncomputable def AnalyticNumberTheory.LargeSieve.chen1973Lemma5M3 (x z4 : ) (lambda : ) :

                              Source M₃: the non-coprime correction removed from M₁.

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                                noncomputable def AnalyticNumberTheory.LargeSieve.chen1973Lemma5M4 (x z4 : ) (lambda : ) :

                                Source M₄ at the finite pre-contour level: the exact progression remainder after subtracting the coprime principal-character term.

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                                  The actual twisted switched source at modulus q; the coefficient remains literally Λ(n) Φ(x/(p₁p₂n)) / log(x/(p₁p₂)).

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                                    theorem AnalyticNumberTheory.LargeSieve.chen1973Lemma5_character_orthogonality {q x a : } [NeZero q] (hxq : x.Coprime q) :
                                    χ : DirichletCharacter q, star (χ x) * χ a = if x = a then q.totient else 0

                                    The pointwise Dirichlet-character orthogonality relation needed to turn a progression condition into a character sum. Unlike a black-box source premise, this finite step is proved directly from Mathlib's orthogonality API.

                                    The old one-conductor signed ledger. It is useful as the inner character majorant, but it is not Chen's p. 117--119 source M₂: that source still has an outer squarefree d-sum and the restriction (p₁p₂,d)=1.

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                                      Compatibility name for the old one-conductor norm ledger. This is only an inner majorant (the shape later denoted N_m after inserting a prime-pair filter), not the source M₂ of Lemma 5. The source-faithful object is chen1973Lemma5M2Source below.

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                                        Legacy API retained for downstream files. Semantically this is the one-conductor inner majorant, not source M₂.

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                                          The literal primitive twist after retaining the source condition (p₁p₂,d)=1. The character conductor is the independent inner variable l.

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                                            Source-faithful M₂ on pp. 117--119: first the outer d weight, then the independent inner conductor l primitive-character sum, with (p₁p₂,d)=1 inside the prime-pair carrier.

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                                              Signed pre-norm form of the same two-layer source. This is the exact finite target for the Selberg/character expansion of the actual M₄ remainder.

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                                                The old one-conductor real-part ledger is bounded by its norm ledger.

                                                Taking norms conductor-by-conductor bounds the signed two-layer source.

                                                Finite M₄ ≤ M₂ connector with the correct source object. The premise is exactly the still-separate Selberg-coefficient/imprimitive-character expansion; the norm step itself is proved here.

                                                The preceding connector specialized to Chen's actual p. 115 Selberg coefficient, making the link to the concrete M₄ remainder explicit.

                                                Exact finite M₁-M₃+M₄ decomposition of the expanded Selberg square.

                                                Equation (5), now as a kernel-checked finite identity.