On hot spots of the torsion function and the ground state of the Laplacian
We compare the maximum points \(P_Ω\) and \(Q_Ω\) of the torsion function and the positive first Dirichlet eigenfunction. We begin with a systematic analytic description of both hot spots over the full range of opening angles of circular sectors: uniqueness, strict monotonicity with respect to the angle, distinct small-angle asymptotics, and explicit limiting equations in the slit-disk limit. We also prove the existence of a coincidence angle and establish arithmetic properties of the limiting coordinates. We then consider bounded convex planar domains with a line of symmetry, and we prove the sharp diameter-normalized identity \[ \sup_Ω \frac{|P_Ω-Q_Ω|}{\operatorname{diam}(Ω)}=\frac12. \] Its upper-bound argument uses standard localization in the heart and an elementary geometric enclosure. Sharpness follows from thin convex domains with a vanishing relative width perturbation peaking near one endpoint: the torsion hot spot approaches that endpoint, whereas the ground-state hot spot approaches the midpoint.