Formal functional equations and generalized Lie-Gröbner series

## Formal functional equations and generalized Lie-Gröbner series

Jointly written with LUDWIG REICH.

Journal of Difference Equations and Applications 24 Issue 5, 1-28 (2017). DOI 10.1080/10236198.2017.1328507

Abstract. Studying the translation equation F(s+t,x)=F(s,F(t,x)), s,t∈ ℂ, for Ft(x)=F(t,x)=ν≥ 1cν(t)xν, t∈ ℂ, or the associated system of cocycle equations in rings of formal power series it is well known that the coefficient functions of their solutions are polynomials in additive and generalized exponential functions. Replacing these functions by indeterminates we obtain formal functional equations. Applying formal differentiation operators to these formal equations we obtain different types of formal differential equations. They can be solved in order to get explicit representations of the coefficient functions. In the present paper we consider iteration groups of type II, i.e. solutions of the translation equation of the form F(t,x)=x+n≥ kcn(t)xn, t∈ ℂ, where k≥ 2 and ck: ℂ→ ℂ is an additive function different from 0. They correspond to formal iteration groups G(y,x)∈ (ℂ[y])[[x]] of type II, which turn out to be the Lie-Gröbner series LGy(x)=n≥ 0(1)/(n!)Dn(x)yn. Here the Lie-Gröbner operator D is defined by D(f(x))=f'(x)H(x) for f∈ C[[x]] where H is the formal generator of G. Using this particular form of the formal iteration group G we are able to find short proofs and elegant representations of the solutions of the cocycle equations. In connection with the second cocycle equation we study the generalized Lie-Gröbner operator D(f)= (j=1k-1jxj)f(x)+f'(x)H(x), f∈ C[[x]], where κ1,…,κk-1∈ ℂ are given. It yields the corresponding generalized Lie-Gröbner series LGy(x)=n≥ 0(1)/(n!)Dn(x)yn which appears in the presentation of the solution of the second cocycle equation.

harald.fripertinger "at" uni-graz.at, May 6, 2024

 GDPR Formal functional equations and generalized Lie-Gröbner series