Padé approximant related to the Wallis formula

Abstract

Abstract Based on the Padé approximation method, in this paper we determine the coefficients a j $a_{j}$ and b j $b_{j}$ such that π = ( ( 2 n ) ! ! ( 2 n − 1 ) ! ! ) 2 { n k + a 1 n k − 1 + ⋯ + a k n k + 1 + b 1 n k + ⋯ + b k + 1 + O ( 1 n 2 k + 3 ) } , n → ∞ , $$ \pi= \biggl(\frac{(2n)!!}{(2n-1)!!} \biggr)^{2} \biggl\{ \frac {n^{k}+a_{1}n^{k-1}+\cdots+a_{k}}{n^{k+1}+b_{1}n^{k}+\cdots+b_{k+1}}+O \biggl(\frac{1}{n^{2k+3}} \biggr) \biggr\} ,\quad n\to\infty, $$ where k ≥ 0 $k\geq0$ is any given integer. Based on the obtained result, we establish a more accurate formula for approximating π, which refines some known results.

Keywords

• gamma function
• psi function
• Wallis ratio
• inequality
• approximation