\documentclass[11pt]{article}
\usepackage[all,knot,poly]{xy}
\DeclareMathSizes{10.95 }{6}{3}{2}
\begin{document}
%{\tiny
\[
\xy
\vtwist~{(-3,3)}{(3,3)}{(-3,-3)}{(3,-3)};
(-12,6)*{1};(12,6)*{n};(-3,6)*{i};(3,6)*{i+1};
(-12,-3)*{}; (-12,3)*{} **\dir{-};
(12,-3)*{}; (12,3)*{} **\dir{-};
(-7,0)*{\dots}; (7,0)*{\dots};
\endxy
\]
%}
\end{document}