$η-$ Hyperbolic Ricci Solitons on the Unit Tangent Bundle of the Hyperbolic Strip

Abstract

We study Killing vector fields, Ricci solitons and hyperbolic Ricci solitons on the hyperbolic strip $S_a=\{(x,y)\in\mathbb{R}^2:0<y<a\}$ endowed with the metric $g_{S_{a}}=\frac{π^2}{a^2\sin^2(πy/a)}(dx^2+dy^2)$, and on its unit tangent bundle $T^1S_a$ endowed with the Sasaki metric $g^S$. On the strip, we show that the Gauss curvature is constant equal to $-1$, that the Lie algebra of Killing fields is isomorphic to $\mathfrak{sl}(2,\mathbb{R})$, that every Ricci soliton is trivial (Killing potential, $λ=-1$), and that a $2$-Killing field which is conformal, or which is the potential of a hyperbolic Ricci soliton with $λ\neq0$, is a Killing field. On the unit tangent bundle, we prove that the Lie algebra of Killing fields of $(T^1S_a,g^S)$ is four-dimensional and isomorphic to $\mathfrak{sl}(2,\mathbb{R})\oplus\mathbb{R}$, that $g^S$ is not Einstein but is $η$-Einstein with constant coefficients, $\operatorname{Ric}^S=-\tfrac32\,g^S+2\,η\otimesη$, and that $(T^1S_a,g^S)$ admits no Ricci soliton. For hyperbolic Ricci solitons we prove non-existence for Killing, left-invariant, and $θ$-invariant potentials with conformal projection, and we reduce the general $θ$-invariant case to a system of equations on the strip; the classification for an arbitrary potential is left open. Finally, we show that the $η$-Ricci solitons of $g^S$ have a Killing potential and $λ=\tfrac32$, $μ=-2$, and that the $η$-hyperbolic Ricci solitons obtained in the same classes of potentials are the trivial ones, with a Killing potential, $μ=-\tfrac32$ and $ν=2$.