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Generalizing the Connes Moscovici Hopf algebra to contain all rooted trees. (English) Zbl 1322.81074

A Hopf algebra \(\mathcal H_{rt}\) which is a generalization of \(\mathcal H(1)\) and contains \(\mathcal H_{rt}\), the Hopf algebra of rooted trees, is defined.
\(\mathcal H_{rt}\) is relevant to understanding of the perturbative regularization of Quantum Field Theory [A. Connes and H. Moscovici, Commun. Math. Phys. 198, No. 1, 199–246 (1998; Zbl 0940.58005)]. Known Hochschild one-cocycle of \(\mathcal H_{rt}\) plays an important role in understanding the combinatirics underlying the Dyson-Schwinger equation [C. Bergbauer and D. Kreimer, in: Physics and number theory. Zürich: European Mathematical Society Publishing House. 133–164 (2006; Zbl 1141.81024)]. This cocycle is described in §3). The authors say knowing about cohomology of \(H_{rt}\) will lead to a better understanding of connection of non-commutative geometry and renormalization.
\(\mathcal H(1)\) is \(n = 1\) case of \(\mathcal H(n)\), a family of Hopf algebras on \(n\)-dimensional flat manifold \(M\) intrduced by Connes and Moscovici [loc. cit.]. They act on the group of smooth functions on the frame bundles with local diffeomorphism on \(M\). They well studied in non-commutative geometry.
The key of the construction of \(\mathcal H(1)_{rt}\) is given in works of A. Cayley [Am. J. Math. 4, 266–268 (1881; JFM 13.0867.02)] and J. C. Butcher [The numerical analysis of ordinary differential equations. Runge-Kutta and general linear methods. Chichester etc.: John Wiley & Sons (1987; Zbl 0616.65072)] (reviewed in §2) that relate rooted trees to a particular first order differential equation. In §3, \(\mathcal H_{rt}\) is defined. It is a graded algebra graded by the number of vertexes of the tree. Then the natural growth operator \(N\) and generalized natural growth operator \(N_t\) are introduced and show \(\mathcal H_{rt}\) is generated by repeated application of operaotrs of the form \(N_t\) to the tree with a single vertex (Theorem 2).
In §4, \(\mathcal H(1)\) is shown to be the bicrossed product of \(\mathcal H_{CK}\), the algebra \(\mathbb Q=\{N^{i-1} (\bullet)|i \in \mathbb N\}\) and the universal enveloping algebra of \(\mathbb{Q}\), generated by two vector fields corresponding grading operator and natural growth operator. Then. in §5, the last one, showing the bicrossed product of \(H_{rt}\) and the universal enveloping algebra of \(\mathfrak{g}_{rt}\) , the Lie algebra generated by vector field \(X_t\), for \(t\) a rooted tree, is isomorphic the bicrossed product of \(\mathcal H_{CK}\) and the universal enveloping algebra of \(\mathbb{Q}\) (Theorem 9), \(\mathcal H(1)_{rt}\) is defined to be this bicrossed product.

MSC:

81T75 Noncommutative geometry methods in quantum field theory
16T05 Hopf algebras and their applications
57T05 Hopf algebras (aspects of homology and homotopy of topological groups)
58B34 Noncommutative geometry (à la Connes)
05C05 Trees
17B22 Root systems
81T15 Perturbative methods of renormalization applied to problems in quantum field theory

References:

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[13] Wulkenhaar, R., “On the Connes Moscovici Hopf algebra associated to the diffeomorphisms of a manifold,” e-print .
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