Poincar\'e inequalities and $A_p$ weights on bow-ties

Abstract

A metric space $X$ is called a \emph{bow-tie} if it can be written as $X=X_{+} \cup X_{-}$, where $X_{+} \cap X_{-}=\{x_0\}$ and $X_{\pm} \ne \{x_0\}$ are closed subsets of $X$. We show that a doubling measure $\mu$ on $X$ supports a $(q,p)$--Poincar\'e inequality on $X$ if and only if $X$ satisfies a quasiconvexity-type condition, $\mu$ supports a $(q,p)$-Poincar\'e inequality on both $X_{+}$ and $X_{-}$, and a variational \p-capacity condition holds. This capacity condition is in turn characterized by a sharp measure decay condition at $x_0$. In particular, we study the bow-tie $X_{\mathbf{R}^n}$ consisting of the positive and negative hyperquadrants in $\mathbf{R}^n$ equipped with a radial doubling weight and characterize the validity of the \p-Poincar\'e inequality on $X_{\mathbf{R}^n}$ in several ways. For such weights, we also give a general formula for the capacity of annuli around the origin.