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Finite Dyson remainder, coefficient unitarity, and regulator counterexamples
Statement
Assume the Countable Choice axiom (The Axiom of Countable Choice ()). Let , , , and . Let be a Hermitian matrix with entries in joules, and let be norm-continuous, Hermitian-valued, and in joules. Define The coupling is dimensionless and is dimensionless. Put and Then the series converges in operator norm on , is the unique solution of , , and is unitary. For every ,
For a finite matrix , define its Hermitian imaginary part by ; for a scalar entry , has its ordinary scalar meaning.
Define and . For every pair of basis indices , exact unitarity gives and coefficient comparison gives and . These are identities for this finite matrix model.
For on , where is in joules and , the evolution matrix is Its exact off-diagonal matrix-element square is ; its first-order squared coefficient is , which can exceed . For every positive rate in , the survival matrix-element square is not .
For an integer , on the finite basis define (zero at ) and for , with . Then No finite-dimensional matrices can satisfy , because the trace of a commutator is zero while the trace of is the dimension.
Facts & Assumptions
Given: , , , real dimensionless , Hermitian , and a norm-continuous Hermitian matrix family on , all with the units fixed in QFT example units, metric, gauge, and pairing conventions.
QFT example units, metric, gauge, and pairing conventions: has units , energy matrices have units joules, is dimensionless, and finite matrices use the operator norm.
Norm Dyson series and uniqueness: for the stated norm-continuous bounded self-adjoint operator family, the Volterra equation has a unique continuous solution.
Norm Dyson series and uniqueness: every propagator supplied by that theorem is unitary; its full statement also supplies the norm-Dyson convergence and factorial bound.
The Axiom of Countable Choice (): is the choice principle assumed by the exact controlled-dynamics supplier; no stronger choice principle is used.
Proof
The matrix exponential is defined by its norm-convergent power series; termwise differentiation on compact time intervals gives for . Hence and , so is unitary (in finite dimension a left inverse is an inverse). Therefore is Hermitian and norm-continuous. Its norm is bounded on compact , so .
Recursion expresses as an -fold ordered integral with integrand of norm at most . The volume of is : induction integrates the preceding volume from to . Thus and the same bound with replaced by any holds uniformly. The scalar exponential series proves uniform absolute convergence.
For , separate the even and odd powers in the absolutely convergent exponential series: where . Its off-diagonal entry has modulus square , and the first-order term has modulus square . Taking makes the latter exceed . Also has derivative at , whereas has derivative there; so they cannot agree for all when .
For the tail, write . Then where an empty product is and . The norm tail is bounded by this scalar tail.
The family satisfies the exact hypotheses of [F2] and [F3] on the compact interval : it is bounded, norm-continuous and self-adjoint, and the required is [A1]. The recursion above is its Dyson recursion, so [F2] supplies the Volterra equation and uniqueness, [F3] supplies unitarity of , and step 1.2 supplies the displayed sharper bound with .
From , expands to . Its entry is , proving the exact identity. The norm-convergent power series makes analytic near (indeed entire), so coefficient comparison is valid: the term gives , and the term gives , hence .
For the linear truncation with , multiplication gives . If , then is nonzero positive semidefinite because ; hence this truncation is not unitary. The exact coefficient relation in step 3.1 therefore requires the higher-order correction whenever its right side is nonzero.
For , direct application gives and , so the commutator equals . At , the two terms are and , so their difference is . These are exactly the values of on each basis vector. For arbitrary finite matrices , by swapping dummy indices, so ; but . Thus the unmodified canonical commutator cannot hold on a finite-dimensional space.