# Members & Guests

## Dr. Paul-Andi Nagy

### Researcher

Universität Stuttgart

## Project

74Rigidity, stability and deformations in nearly parallel G2-geometry

## Publications within SPP2026

We obtain new lower bounds for the first non-zero eigenvalue of the scalar

sub-Laplacian for 3-Sasaki metrics, improving Lichnerowicz-Obata type

estimates by Ivanov et al. The limiting eigenspace is fully decribed in

terms of the automorphism algebra. Our results can be thought of as an

analogue of the Lichnerowicz-Matsushima estimate for Kähler-Einstein

metrics. In dimension 7, if the automorphism algebra is non-vanishing,

we also compute the second eigenvalue for the sub-Laplacian and construct

explicit eigenfunctions. In addition, for all metrics in the canonical

variation of the 3-Sasaki metric we give a lower bound for the spectrum of the Riemannian Laplace operator, depending only on scalar curvature and dimension.

We also strengthen a result pertaining to the growth rate of harmonic

functions, due to Conlon, Hein and Sun, in the case of hyperkähler

cones. In this setup we also describe the space of holomorphic functions.

Related project(s):
74Rigidity, stability and deformations in nearly parallel G2-geometry

We initiate a systematic study of the deformation theory of the second Einstein

metric $$g_{1/\sqrt{5}}$$  respectively the proper nearly G2 structure  $$\phi_{1/\sqrt{5}}$$ of a 3-Sasaki manifold $$(M^7,g)$$. We show that infinitesimal Einstein deformations for  $$g_{1/\sqrt{5}}$$   coincide with infinitesimal  $$G_2$$ deformations for $$\phi_{1/\sqrt{5}}$$. The latter are showed to be parametrised by eigenfunctions of the basic Laplacian of g, with eigenvalue twice the Einstein constant of the base 4-dimensional orbifold, via an explicit differential operator. In terms of this parametrisation we determine those infinitesimal $$G_2$$ deformations which are unobstructed to second order.