a | b conditional π π {\displaystyle{\displaystyle a|b}} a β£ b conditional π π {\displaystyle{\displaystyle a\mid b}} 2 β’ n !! 2 double-factorial π {\displaystyle{\displaystyle 2n!!}} β n = 0 β 1 n + 1 + n = n superscript subscript π 0 1 π 1 π π {\displaystyle{\displaystyle\sum_{n=0}^{\infty}\frac{1}{n}+1+n=n}}