Nilpotent Hessenberg varieties are a family of subvarieties of the flag variety, which include the Springer varieties, the Peterson variety, and the whole flag variety. In this thesis I give a geometric proof that the cohomology of the flag variety surjects onto the cohomology of the Peterson variety; I provide a combinatorial criterion for determing the singular loci of a large family of regular nilpotent Hessenberg varieties; and I describe the equivariant cohomology of any regular nilpotent Hessenberg variety whose cohomology is generated by its degree two classes.

Accurate interatomic energies and forces enable high-quality molecular dynamics simulations, torsion scans, potential energy surface mapping, and geometry optimization. Machine learning algorithms have enabled rapid estimates of energies and forces with high accuracy. Further development of machine learning algorithms holds promise for producing general potentials that support dozens of atomic species. I present my own Transformer Interatomic Potential (TrIP): a chemically sound potential based on the SE(3)-Transformer. TrIP's species-agnostic architecture--using continuous atomic representation and homogenous graph convolutions--encourages parameter sharing between atomic species for more general representations of chemical environments, keeps a reasonable number of parameters, serves as a form of regularization, and is a step towards accurate universal interatomic potentials. I introduce physical bias in the form of Ziegler-Biersack-Littmark-screened nuclear repulsion and constrained atomization energies to improve qualitative behavior for near and far interaction. TrIP achieves state-of-the-art accuracies on the COMP6 benchmark with an energy prediction error of just 1.02 kcal/mol MAE, outperforming all other models. An energy scan of a water molecule shows improved short- and long-range interactions compared to other neural network potentials, demonstrating its physical realism compared to other models. TrIP also shows stability in molecular dynamics simulations with a reasonable exploration of Ramachandran space.

Let G be a finite group and V be an orthogonal four-dimensional real representation space of G where the action of G is non-free. We give necessary and sufficient conditions for the existence of a G-equivariant vector field on the representation sphere of V in the cases G is the dihedral group, the generalized quaternion group and the semidihedral group in terms of decomposition of V into irreducible representations. In the case G is abelian, where the solution is already known, we give a more elementary solution.

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Let $M$ be a smooth compact manifold equipped with a co-oriented subbundle $E\subset TM$. We suppose that a compact Lie group $G$ acts on $M$ preserving $E$, such that the $G$-orbits are transverse to $E$. If the fibres of $E$ are equipped with a complex structure then it is possible to construct a $G$-invariant Dirac operator $\dirac$ in terms of the resulting almost CR structure. We show that there is a canonical equivariant differential form with generalized coefficients $\mathcal{J}(E,X)$ defined on $M$ that depends only on the $G$-action and the co-oriented subbundle $E$. Moreover, the group action is such that $\dirac$ is a $G$-transversally elliptic operator in the sense of Atiyah \cite{AT}. Its index is thus defined as a generalized function on $G$. Beginning with the equivariant index formula of Paradan and Vergne \cite{PV3}, we obtain an index formula for $\dirac$ computed as an integral over $M$ that is free of choices and growth conditions. This formula necessarily involves equivariant differential forms with generalized coefficients and we show that the only such form required is the canonical form $\mathcal{J}(E,X)$. In certain cases the index of $\dirac$ can be interpreted in terms of a CR analogue of the space of holomorphic sections, allowing us to view our index formula as a character formula for the $G$-equivariant quantization of the almost CR manifold $(M,E)$. In particular, we obtain the ``almost CR'' quantization of a contact manifold, in a manner directly analogous to the almost complex quantization of a symplectic manifold.

Let $M$ be a smooth compact manifold equipped with a co-oriented subbundle $E\subset TM$. We suppose that a compact Lie group $G$ acts on $M$ preserving $E$, such that the $G$-orbits are transverse to $E$. If the fibres of $E$ are equipped with a complex structure then it is possible to construct a $G$-invariant Dirac operator $\dirac$ in terms of the resulting almost CR structure. We show that there is a canonical equivariant differential form with generalized coefficients $\mathcal{J}(E,X)$ defined on $M$ that depends only on the $G$-action and the co-oriented subbundle $E$. Moreover, the group action is such that $\dirac$ is a $G$-transversally elliptic operator in the sense of Atiyah \cite{AT}. Its index is thus defined as a generalized function on $G$. Beginning with the equivariant index formula of Paradan and Vergne \cite{PV3}, we obtain an index formula for $\dirac$ computed as an integral over $M$ that is free of choices and growth conditions. This formula necessarily involves equivariant differential forms with generalized coefficients and we show that the only such form required is the canonical form $\mathcal{J}(E,X)$. In certain cases the index of $\dirac$ can be interpreted in terms of a CR analogue of the space of holomorphic sections, allowing us to view our index formula as a character formula for the $G$-equivariant quantization of the almost CR manifold $(M,E)$. In particular, we obtain the ``almost CR'' quantization of a contact manifold, in a manner directly analogous to the almost complex quantization of a symplectic manifold.

For T the circle group, we construct a differential refinement of T-equivariant K-theory. We first construct a de Rham model for delocalized equivariant cohomology and a delocalized equivariant Chern character based on [19] and [14]. We show that the delocalized equivariant Chern character induces a complex isomorphism. We then construct a geometric model for differential T-equivariant K-theory analogous to the model of differential K-theory in [27] and deduce its basic properties. / text

The construction of characteristic classes via the curvature form of a connection is one motivation for the refinement of integral cohomology by de Rham cocycles -- known as differential cohomology. We will discuss the analog in the case of a group action on the manifold: We will show the compatibility of the equivariant characteristic class in integral Borel cohomology with the equivariant characteristic form in the Cartan model. Motivated by this understanding of characteristic forms, we define equivariant differential cohomology as a refinement of equivariant integral cohomology by Cartan cocycles.

This thesis is devoted to a problem in the representation theory of the symmetric group over C (the field of the complex numbers). Let d be a positive integer, and let S_d denote the symmetric group on d letters. Given a partition k of d, the Specht module V_k is a finite dimensional vector space over C which admits a natural basis indexed by all standard tableaux of shape k with entries in {1, 2, ..., d}. It affords an irreducible representation of the symmetric group S_d, and conversely every irreducible representation of S_d is isomorphic to V_k for some partition k. Given two Specht modules V_k, V_t their tensor product representation is in general reducible, and hence it splits into a direct sum of irreducibles. This raises the problem of describing the S_d equivariant projection morphisms (alternately called S_d-homomorphisms) in terms of the standard tableaux basis. In this work we give explicit formulae describing this morphism in the following cases: k=(d-1, 1), (d-2, 1,1), (2, 1,... ,1). Finally, we present a conjecture formula for the q-morphism in the case k=(d-r, 1, ..., 1).

This thesis is devoted to a problem in the representation theory of the symmetric group over C (the field of the complex numbers). Let d be a positive integer, and let S_d denote the symmetric group on d letters. Given a partition k of d, the Specht module V_k is a finite dimensional vector space over C which admits a natural basis indexed by all standard tableaux of shape k with entries in {1, 2, ..., d}. It affords an irreducible representation of the symmetric group S_d, and conversely every irreducible representation of S_d is isomorphic to V_k for some partition k. Given two Specht modules V_k, V_t their tensor product representation is in general reducible, and hence it splits into a direct sum of irreducibles. This raises the problem of describing the S_d equivariant projection morphisms (alternately called S_d-homomorphisms) in terms of the standard tableaux basis. In this work we give explicit formulae describing this morphism in the following cases: k=(d-1, 1), (d-2, 1,1), (2, 1,... ,1). Finally, we present a conjecture formula for the q-morphism in the case k=(d-r, 1, ..., 1).