Belyi-extending maps and the Galois action on dessins d'enfants

We study the absolute Galois group by looking for invariants and orbits of its faithful action on Grothendieck's dessins d'enfants. We define a class of functions called Belyi-extending maps, which we use to construct new Galois invariants of dessins from previously known invariants. Belyi-extending maps are the source of the ``new-type'' relations on the injection of the absolute Galois group into the Grothendieck-Teichmuller group. We make explicit how to get from a general Belyi-extending map to formula for its associated invariant which can be implemented in a computer algebra package. We give an example of a new invariant differing on two dessins which have the same values for the other readily computable invariants.Comment: 13 pages, 7 figures; submitted for publication; revisions are that the paper now deals only with Galois invariants of dessins, and that material is slightly expande

Intergenerational equity and stationarity

We consider quasi-orderings of infinite utility streams satisfying the strong Pareto axiom (i.e., Paretian quasi-orderings) and study the question of how strong a notion of intergenerational equity one can impose on these quasi-orderings without generating an impossibility theorem. Building on a result by Mitra and Basu (2007), we first show that there exist many possible extensions of the finite anonymity axiom that are satisfied by some Paretian quasiordering. Then we study how the additional requirement of stationarity `a la Koopmans (1960) affects this result. After proving a possibility theorem for this case, we demonstrate that stationarityimposes strong restrictions on the extendability of the finite anonymity axiom.

Wada Dessins associated with Finite Projective Spaces and Frobenius Compatibility

\textit{Dessins d'enfants} (hypermaps) are useful to describe algebraic properties of the Riemann surfaces they are embedded in. In general, it is not easy to describe algebraic properties of the surface of the embedding starting from the combinatorial properties of an embedded dessin. However, this task becomes easier if the dessin has a large automorphism group. In this paper we consider a special type of dessins, so-called \textit{Wada dessins}. Their underlying graph illustrates the incidence structure of finite projective spaces \PR{m}{n}. Usually, the automorphism group of these dessins is a cyclic \textit{Singer group} $\Sigma_\ell$ permuting transitively the vertices. However, in some cases, a second group of automorphisms $\Phi_f$ exists. It is a cyclic group generated by the \textit{Frobenius automorphism}. We show under what conditions $\Phi_f$ is a group of automorphisms acting freely on the edges of the considered dessins.Comment: 23 page

On the leximin and utilitarian overtaking criteria with extended anonymity

This paper studies the extensions of the infinte-horizon variants of the leximin principle and utilitarianism on the set of infinite utility streams. We especially examine those extensions which satisfy the axiom of Preference-continuity (or Consistency) and the extended anonymity axiom called Q-Anonymity. We formulate new extended leximin and utilitarian social welfare relations (SWRs), called Q-W-leximin SWR and Q-overtaking criterion respectively, and show that Weak Preference-continuity (or Weak Consistency) and Q-Anonymity together with Strong Pareto and Hammond Equity (resp. Partial Unit Comparability) characterize all SWRs that include the Q-W-leximin SWR (resp. the Q-overtaking criterion) as a subrelation. We also show that there exists no SWR satisfying Strong Pareto, Strong Preference-continuity (or Strong Consistency) and Q-Anonymity.Q-Anonymity, Preference-continuity, Consistency, Leximin, Utilitarianism, Overtaking criterion