Wallis product

(mathematics) An infinite product that approximates half of pi: 𝜋/2=∏ₙ₌₁ ᪲(4n²)/(4n²-1)=∏ₙ₌₁ ᪲((2n)/(2n-1)·(2n)/(2n+1))\[6pt]= Big (2/1·2/3 Big )· Big (4/3·4/5 Big )· Big (6/5·6/7 Big )· Big (8/7·8/9 Big )·;⋯\.