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Arpan Saha

Publications and source records attributed to Arpan Saha.

11 recordsLinked to original sources
arXiv2025

Vortex Dynamics in Tubular Fluid Membranes

Thin cylindrical membranes arise in a wide variety of biological systems ranging from tubular structures on and within cell membranes to in-vitro experiments on artificial vesicles. Motor proteins embedded in such fluidic membranes often induce vortex-like flows. In this work, we construct a class of 2D vortex flow in a thin tubular membrane, coupled to 3D external embedding fluids. The cylinder topology enforces the creation of an additional saddle in the flow field, consistent with Poincaré Index Theorem. In this setup, the incompressibility of the membrane fluid can be utilized to cast the dynamics of a multi-vortex system in the form of a Hamiltonian, This Hamiltonian also incorporates the specific couplings of the 2D membrane flow with the 3D external fluids. The cylinder geometry breaks the in-plane rotational symmetry of the membrane and leads to several interesting features in the multi-vortex dynamics, such as orbit pinching, For a two-vortex system of same circulation, we observe closed orbits with the inter-vortex separation oscillating in time, unlike flat and spherical fluid membranes, where the separation remains constant. Vortex pairs (vortices with opposite circulation) move together along helical geodesics in accordance with a conjecture by Kimura, Proceedings of the Royal Society A, Vol 455 (1999), now extended to tubular geometries. We also explore relative equilibria of multi-vortex systems in this setup and demonstrate vortex leapfrogging via numerical simulations. Our results will be interesting in the context of microfluidic flows arising in nature as well as experimental studies in membrane tubes similar to PNAS 108 (31) 12605-12610 (2011).

physics.flu-dyn↗
arXiv2022

A Hyperkähler geometry associated to the BPS structure of the resolved conifold

We associate to the resolved conifold an affine special Kähler (ASK) manifold of complex dimension 1, and an instanton corrected hyperkähler (HK) manifold of complex dimension 2. We describe these geometries explicitly, and show that the instanton corrected HK geometry realizes an Ooguri-Vafa-like smoothing of the semi-flat HK metric associated to the ASK geometry. On the other hand, the instanton corrected HK geometry associated to the resolved conifold can be described in terms of a twistor family of two holomorphic Darboux coordinates. We study a certain conformal limit of the twistor coordinates, and conjecture a relation to a solution of a Riemann-Hilbert problem previously considered by T. Bridgeland.

math.DG↗
arXiv2022

On the integrable hierarchy for the resolved conifold

We provide a direct proof of a conjecture of Brini relating the Gromov-Witten theory of the resolved conifold to the Ablowitz-Ladik integrable hierarchy at the level of primaries. In doing so, we use a functional representation of the Ablowitz-Ladik hierarchy as well as a difference equation for the Gromov-Witten potential. In particular, we express certain distinguished solutions of the difference equation in terms of an analytic function which is a specialization of a Tau function put forward by Bridgeland in the study of wall-crossing phenomena of Donaldson-Thomas invariants.

math.AG↗
arXiv2022

The c-map as a functor on certain variations of Hodge structure

We give a new manifestly natural presentation of the supergravity c-map. We achieve this by giving a more explicit description of the correspondence between projective special Kähler manifolds and variations of Hodge structure, and by demonstrating that the twist construction of Swann, for a certain kind of twist data, reduces to a quotient by a discrete group. We combine these two ideas by showing that variations of Hodge structure give rise to the aforementioned kind of twist data and by then applying the twist realisation of the c-map due to Macia and Swann. This extends previous results regarding the lifting of general isomorphisms along the undeformed c-map, and of infinitesimal automorphisms along the deformed c-map. We show in fact that general isomorphisms can be naturally lifted along the deformed c-map.

math.DG↗
arXiv2021

Mathematical structures of non-perturbative topological string theory: from GW to DT invariants

We study the Borel summation of the Gromov-Witten potential for the resolved conifold. The Stokes phenomena associated to this Borel summation are shown to encode the Donaldson-Thomas invariants of the resolved conifold, having a direct relation to the Riemann-Hilbert problem formulated by T. Bridgeland. There exist distinguished integration contours for which the Borel summation reproduces previous proposals for the non-perturbative topological string partition functions of the resolved conifold. These partition functions are shown to have another asymptotic expansion at strong topological string coupling. We demonstrate that the Stokes phenomena of the strong-coupling expansion encode the DT invariants of the resolved conifold in a second way. Mathematically, one finds a relation to Riemann-Hilbert problems associated to DT invariants which is different from the one found at weak coupling. The Stokes phenomena of the strong-coupling expansion turn out to be closely related to the wall-crossing phenomena in the spectrum of BPS states on the resolved conifold studied in the context of supergravity by D. Jafferis and G. Moore.

hep-th↗
arXiv2021

Four-dimensional Einstein manifolds with Heisenberg symmetry

We classify Einstein metrics on $\mathbb{R}^4$ invariant under a four-dimensional group of isometries including a principal action of the Heisenberg group. The metrics are either Ricci-flat or of negative Ricci curvature. We show that all of the Ricci-flat metrics, including the simplest ones which are hyper-Kähler, are incomplete. By contrast, those of negative Ricci curvature contain precisely two complete examples: the complex hyperbolic metric and a metric of cohomogeneity one known as the one-loop deformed universal hypermultiplet.

math.DG↗
arXiv2017

Quarter-pinched Einstein metrics interpolating between real and complex hyperbolic metrics

We show that the one-loop quantum deformation of the universal hypermultiplet provides a family of complete $1/4$-pinched negatively curved quaternionic Kähler (i.e. half conformally flat Einstein) metrics $g^c$, $c\ge 0$, on $\mathbb R^4$. The metric $g^0$ is the complex hyperbolic metric whereas the family $(g^c)_{c>0}$ is equivalent to a family of metrics $(h^b)_{b>0}$ depending on $b=1/c$ and smoothly extending to $b=0$ for which $h^0$ is the real hyperbolic metric. In this sense the one-loop deformation interpolates between the real and the complex hyperbolic metrics. We also determine the (singular) conformal structure at infinity for the above families.

math.DG↗
arXiv2014

Interpolating from Bianchi Attractors to Lifshitz and AdS Spacetimes

We construct classes of smooth metrics which interpolate from Bianchi attractor geometries of Types II, III, VI and IX in the IR to Lifshitz or $AdS_2 \times S^3$ geometries in the UV. While we do not obtain these metrics as solutions of Einstein gravity coupled to a simple matter field theory, we show that the matter sector stress-energy required to support these geometries (via the Einstein equations) does satisfy the weak, and therefore also the null, energy condition. Since Lifshitz or $AdS_2 \times S^3$ geometries can in turn be connected to $AdS_5$ spacetime, our results show that there is no barrier, at least at the level of the energy conditions, for solutions to arise connecting these Bianchi attractor geometries to $AdS_5$ spacetime. The asymptotic $AdS_5$ spacetime has no non-normalizable metric deformation turned on, which suggests that furthermore, the Bianchi attractor geometries can be the IR geometries dual to field theories living in flat space, with the breaking of symmetries being either spontaneous or due to sources for other fields. Finally, we show that for a large class of flows which connect two Bianchi attractors, a C-function can be defined which is monotonically decreasing from the UV to the IR as long as the null energy condition is satisfied. However, except for special examples of Bianchi attractors (including AdS space), this function does not attain a finite and non-vanishing constant value at the end points.

hep-th↗
arXiv2012

Gauging gravity with SO(1,3) for spin-1/2 particles

We demonstrate, by analogy with electromagnetism, that the geometric content in the theory of gravity is an indirect consequence of the fact that the gauge group in question is the Lorentz group SO(1,3). We hence construct field equations for gravity and a spin-1/2 particle in a gravitational field based on gauge considerations. Furthermore, we derive the weak field and Schroedinger limits of the Dirac equation of the particle in the gravitational field, especially in Fermi normal coordinates and on the equatorial plane of the Kerr geometry, following which we identify the terms to which the electromagnetic potentials A and Phi are analogous.

gr-qc↗
arXiv2011

A Few Equivalences of Wall-Sun-Sun Prime Conjecture

In this paper, we prove a few lemmas concerning Fibonacci numbers modulo primes and provide a few statements that are equivalent to Wall-Sun-Sun Prime Conjecture. Further, we investigate the conjecture through heuristic arguments and propose a few additional conjectures for future research.

math.NT↗