Duality Groups, Automorphic Forms and Higher Derivative Corrections

We study the higher derivative corrections that occur in type II superstring theories in ten dimensions or less. Assuming invariance under a discrete duality group G(Z) we show that the generic functions of the scalar fields that occur can be identified with automorphic forms. We then give a systematic method to construct automorphic forms from a given group G(Z) together with a chosen subgroup H and a linear representation of G(Z). This construction is based on the theory of non-linear realizations and we find that the automorphic forms contain the weights of G. We also carry out the dimensional reduction of the generic higher derivative corrections of the IIB theory to three dimensions and find that the weights of E_8 occur generalizing previous results of the authors on M-theory. Since the automorphic forms of this theory contain the weights of E_8 we can interpret the occurrence of weights in the dimensional reduction as evidence for an underlying U-duality symmetry.Comment: Typos corrected and a reference adde

Enhanced Coset Symmetries and Higher Derivative Corrections

After dimensional reduction to three dimensions, the lowest order effective actions for pure gravity, M-theory and the Bosonic string admit an enhanced symmetry group. In this paper we initiate study of how this enhancement is affected by the inclusion of higher derivative terms. In particular we show that the coefficients of the scalar fields associated to the Cartan subalgebra are given by weights of the enhanced symmetry group.Comment: 37 pages Latex. Reference added and other minor correction

Perturbation Theory From Automorphic Forms

Using our previous construction of Eisenstein-like automorphic forms we derive formulae for the perturbative and non-perturbative parts for any group and representation. The result is written in terms of the weights of the representation and the derivation is largely group theoretical. Specialising to the E_{n+1} groups relevant to type II string theory and the representation associated with node n+1 of the E_{n+1} Dynkin diagram we explicitly find the perturbative part in terms of String Theory variables, such as the string coupling g_d and volume V_n. For dimensions seven and higher we find that the perturbation theory involves only two terms. In six dimensions we construct the SO(5,5) automorphic form using the vector representation. Although these automorphic forms are generally compatible with String Theory, the one relevant to R^4 involves terms with g_d^{-6} and so is problematic. We then study a constrained SO(5,5) automorphic form, obtained by summing over null vectors, and compute its perturbative part. We find that it is consistent with String Theory and makes precise predictions for the perturbative results. We also study the unconstrained automorphic forms for E_6 in the 27 representation and E_7 in the 133 representation, giving their perturbative part and commenting on their role in String Theory.Comment: Typos fixed and other minor corrections. A 'note added' include

Bridge trisections in rational surfaces

We study smooth isotopy classes of complex curves in complex surfaces from the perspective of the theory of bridge trisections, with a special focus on curves in $\mathbb{CP}^2$ and $\mathbb{CP}^1\times\mathbb{CP}^1$. We are especially interested in bridge trisections and trisections that are as simple as possible, which we call "efficient". We show that any curve in $\mathbb{CP}^2$ or $\mathbb{CP}^1\times\mathbb{CP}^1$ admits an efficient bridge trisection. Because bridge trisections and trisections are nicely related via branched covering operations, we are able to give many examples of complex surfaces that admit efficient trisections. Among these are hypersurfaces in $\mathbb{CP}^3$, the elliptic surfaces $E(n)$, the Horikawa surfaces $H(n)$, and complete intersections of hypersurfaces in $\mathbb{CP}^N$. As a corollary, we observe that, in many cases, manifolds that are homeomorphic but not diffeomorphic have the same trisection genus, which is consistent with the conjecture that trisection genus is additive under connected sum. We give many trisection diagrams to illustrate our examples.Comment: 46 pages, 28 color figure

Horizontal Inequity can be a Good Thing

A switch from any given income tax schedule to a differentiated tax structure in which two groups of taxpayers are treated differently, each still facing the same local degree of progression, can induce an increase in welfare despite causing horizontal inequity. We demonstrate this result in a number of special case and make a general conjecture, the thrust of which is that society's acceptance of horizontal inequity will be second-best whenever the government must operate with a limited bundle of income tax instruments such as allowances, thresholds and marginal rates.

The Gini coefficient reveals more.

We revisit the well-known decomposition of the Gini coefficient into betweengroups, within-groups and overlap terms in the context of two groups in which the incomes in one group may be scaled and that group’s population weight modified. In this more general setting than usual, we focus on the properties of the overlap term, proving inter alia that overlap unambiguously reduces as a result of a within-group progressive transfer, and is increased by scaling up the incomes in the group with the lower mean, reaching a maximum when the two means become the same. In the case of a socially heterogeneous population and equivalized incomes, the effect on the Gini overlap of changing the income unit is determined, along with that of adjusting the equivalence scale deflator in case the income unit is the equivalent adult (such adjustment simultaneously changing the weighting of income units).