Drawing a Hasse diagram in latex

Here is a way to do it:

documentclass{article}
usepackage{amssymb} 
usepackage{tikz-cd}

begin{document}

begin{tikzcd}[row sep=1.5cm, column sep=1.7cm]
& A 
{a, b}urar & {a, c}uar arrow[from=dr]& {b, c}ular 
{a }uarurar & {b}ular[crossing over] urar[crossing over] & { c} uar
 & varnothingularuar urar
  end{tikzcd} %

end{document}