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The binomial family of the splitting pullback is multiplicative and bracket-preserving, and its logarithm is a derivation
Statement
Let , let be analytic splitting data, and use all notation of The truncated binomial family , the truncated logarithm , its coefficient fields and the truncations : the domains and , the parameters , , the binomial coefficients , the difference operator , the truncated family and the truncated logarithm . Let be holomorphic on , and put . For every and finite , joint local holomorphy of and give a neighborhood of on which the first iterates are defined and stay in . Define from this local holomorphic family. Compatible flow germs make these coefficients independent of the neighborhood; coefficient extraction makes them holomorphic on . On they agree with the uniform construction at order for , where and . Whenever denotes a distance domain below, allocate it with order ; thus even for . For and define the coefficient polynomial
where , and is a polynomial of degree at most in with coefficients in the holomorphic functions on .
Then:
- Integer case. For all integers , and every , (omit the zero summands with to extend the finite sum) on the domain allocated with order .
- Multiplicativity and bracket identity. For every and every , as identities of holomorphic functions on ,
- The logarithm is a derivation. Put (the derivative in at ), so that for the formal logarithm . Then , each is a -linear local differential operator on holomorphic functions, and as functions on ; equivalently the coefficient of in and in obeys the product rule and the bracket derivation rule. Moreover for -degree agrees with the -th coefficient of the concrete holomorphic function restricted to whenever .
Moreover for : each coefficient derivation is the coordinate vector field used in the later Hamiltonian construction.
Facts & Assumptions
Given: The analytic splitting data and all notational conventions of The truncated binomial family , the truncated logarithm , its coefficient fields and the truncations ; holomorphic on .
Binomial algebra: the polynomial identity for holds for the single operator in the algebra of operators on functions holomorphic on the appropriate domains, because all powers of commute; for integers ; and for with value for (The truncated binomial family , the truncated logarithm , its coefficient fields and the truncations , The factorial and the falling factorial , defined by recursion in ).
Pullback calculus: , and for holomorphic , by the chain rule and Hamiltonian flows and the exact splitting map preserve the complexified canonical form and the Poisson bracket clause 3; all maps involved are holomorphic (Calculus of holomorphic maps: chain rule, Wirtinger derivatives, coefficient extraction in one variable, and locally uniform limits, Complex phase domains, holomorphic maps on , and the complexified canonical form and bracket).
Domains: for fixed truncation order and , and for ; is holomorphic on for (The difference operator loses a controlled domain margin and is bounded by ).
Holomorphic functions of : if is holomorphic for and for all such , then all Taylor coefficients at agree; and a polynomial in vanishing at vanishes identically (Calculus of holomorphic maps: chain rule, Wirtinger derivatives, coefficient extraction in one variable, and locally uniform limits clause 3, Iterated Cauchy formula, power-series expansion and Cauchy estimates for functions continuous and separately holomorphic on a polydisc).
Chain rule as applied on this page: the pullback of a bracket and of a product of holomorphic functions is computed by differentiating the composite map, so the complexified bracket of Complex phase domains, holomorphic maps on , and the complexified canonical form and bracket is transformed by the chain rule (The chain rule for total derivatives: , The total (Fréchet) derivative as the linear first-order approximation with remainder, Directional derivatives and partial derivatives of a map , A phase domain, its canonical one-form and its standard symplectic two-form).
Proof
Local coefficients and integer case. On any sufficiently small common neighborhood, write with holomorphic . If a holomorphic family is , then is : factor and integrate the spatial derivative of the remaining family along the short segment from to . Induction therefore gives locally. We prove for and by induction on . For , . If it holds for , then by [L1] on the relevant domains and applied to equals ; applying once more gives by [L2], while the binomial expansion over reproduces because for .
Multiplicativity and bracket identities at integer . For an integer with , both and equal on by step 1.1; likewise and both equal by step 1.1 and [L2]. At any , the same finite pullback identities hold as local germs for arbitrary fixed integers with . Comparing Taylor coefficients in of order on that neighborhood by [L4] and using that contributes only at order gives, for every and every integer (choose an integer truncation , distinct from the field bound in the splitting data),
Polynomial extension. Both sides of each identity of step 2.1 are polynomials in of degree at most (for the -coefficient, products of polynomials of degrees with ), and they agree at every integer ; hence by [L4] they agree for every . This is clause 2.
Derivative at . Differentiate the identities of clause 2 in at and use [L1]: since and for (all coefficients vanish for ), the product identity becomes , and the bracket identity becomes . By [L1] again, . Finally, because the two series differ by terms of order ; hence for the coefficients agree, and is holomorphic on by The difference operator loses a controlled domain margin and is bounded by clause 3, so its Taylor coefficients are the .
Coordinate field identification. Fix . By the integer pullback identity, each is a finite linear combination of with ; for fixed , is holomorphic in near , equals at , and differs from by . The Taylor expansion of at through total degree has a remainder , so replacing by that Taylor polynomial changes none of the coefficients for . The product derivation rule of step 4.1 gives and annihilates at every monomial of degree at least two in : split such a monomial into two factors vanishing at . Only the linear Taylor terms remain, yielding . Since was arbitrary, this is the claimed local differential-operator formula on .
Conclusion. Clause 1 is step 1.1, clause 2 is step 3.1 and clause 3 is step 4.1 together with the -linearity of -differentiation at and of the coefficient extraction.
Remarks
- The key point is that the family is a polynomial in at each -degree, so the integer- identities — which are the multiplicativity and bracket naturality of the pullback — extend to all complex ; differentiating at then produces the derivation identities used to build the modified Hamiltonian. No convergence of the full series is asserted here: only the concrete truncated functions for and their coefficients of order are used.
- All identities are on complex domains; no real-valuedness or physical interpretation is involved, and no choice principle is used.
Depends on
The truncated binomial family , the truncated logarithm , its coefficient fields and the truncations
The difference operator loses a controlled domain margin and is bounded by
Hamiltonian flows and the exact splitting map preserve the complexified canonical form and the Poisson bracket
The flow of a bounded holomorphic field on a polydisc: existence in complex time, displacement bound, holomorphy, and the composition law
Calculus of holomorphic maps: chain rule, Wirtinger derivatives, coefficient extraction in one variable, and locally uniform limits
Iterated Cauchy formula, power-series expansion and Cauchy estimates for functions continuous and separately holomorphic on a polydisc
Analytic splitting data with uniform holomorphic neighbourhood, and the exact symmetric splitting map
Complex phase domains, holomorphic maps on , and the complexified canonical form and bracket
A phase domain, its canonical one-form and its standard symplectic two-form
The chain rule for total derivatives:
The total (Fréchet) derivative as the linear first-order approximation with remainder
Directional derivatives and partial derivatives of a map
The factorial and the falling factorial , defined by recursion in
Dependency tree · 2 levels
Sources
- E. Hairer, Geometric Numerical Integration (TU Munchen 2010), Lecture 3 'Backward error analysis', Section 1: the logarithm of a one-step method as a formal series in the step size, printed pages 1-3
- J. Lebl, Tasty Bits of Several Complex Variables, Chapter 1, Section 1.2: coefficient extraction and polynomial identities for power series in one variable, printed pages 18-23