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BI Airy Bi

BI.1 Introduction

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Let $x$ be a complex variable of $\mathbb{C} \setminus \{\infty\}$ .The function Airy Bi (noted $\operatorname{Bi}$ ) is defined by the following second order differential equation


\begin{equation*} 
\begin{split} 
-x y (x) + \frac{\partial^{2} y (x)}{\partial x^{2}}& =0. 
\end{split} 
\end{equation*}
BI.1.1

The initial conditions of BI.1.1 are given at $0$ by


\begin{equation*} 
\begin{split} 
\operatorname{Bi} (0)& =\frac{3^{\frac{5}{6}}}{3 \Gamma \Bigl(\frac{2}{3}\Bigr)}, \\ 
\frac{\partial \operatorname{Bi} (x)}{\partial x} (0)& =\frac{3^{\frac{2}{3}} \Gamma \Bigl(\frac{2}{3}\Bigr)}{2 \pi}. 
\end{split} 
\end{equation*} 
 BI.1.2

Related function: Airy Ai

BI.2 Series and asymptotic expansions

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BI.2.1 Taylor expansion at $0$

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BI.2.1.2 General form

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\begin{equation*} 
\begin{split} 
\operatorname{Bi} (x)& =\sum_{n = 0}^{\infty} u (n) x^{n}. 
\end{split} 
\end{equation*} 
 BI.2.1.2.1
The coefficients $u (n)$ satisfy the recurrence

\begin{equation*} 
\begin{split} 
-u (n) + \bigl(n^{2} + 5 n + 6\bigr) u (n + 3)& =0. 
\end{split} 
\end{equation*}
BI.2.1.2.2
Initial conditions of BI.2.1.2.2 are given by

\begin{equation*} 
\begin{split} 
u (0)& =\frac{3^{\frac{5}{6}}}{3 \Gamma \Bigl(\frac{2}{3}\Bigr)}, \\ 
u (1)& =\frac{3^{\frac{2}{3}} \Gamma \Bigl(\frac{2}{3}\Bigr)}{2 \pi}, \\ 
u (2)& =0. 
\end{split} 
\end{equation*}
BI.2.1.2.3
The recurrence BI.2.1.2.2 has the closed form solution

\begin{equation*} 
\begin{split} 
u (3 n + 2)& =0, \\ 
u (3 n)& =\frac{3^{\bigl(\frac{5}{6} - 2 n\bigr)}}{3 \Gamma (n + 1) \Gamma \Bigl(n + \frac{2}{3}\Bigr)}, \\ 
u (3 n + 1)& =\frac{3^{\bigl(\frac{1}{6} - 2 n\bigr)}}{3 \Gamma \Bigl(n + \frac{4}{3}\Bigr) \Gamma (n + 1)}. 
\end{split} 
\end{equation*}
BI.2.1.2.4

BI.2.2 Asymptotic expansion at $\infty$

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BI.2.2.1 First terms

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\begin{equation*} 
\begin{split} 
& \operatorname{Bi} (x)\approx \operatorname{e} ^{\biggl(\frac{-2}{3 \xi^{3}}\biggr)} \sqrt{\xi} \Biggl(\frac{-i}{\sqrt{\pi}} + \frac{5 i \xi^{3}}{48 \sqrt{\pi}} + \ldots\Biggr) 
\end{split} 
\end{equation*}
where $\xi = -\sqrt{\frac{1}{x}}$

BI.2.2.2 General form

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\begin{equation*} 
\begin{split} 
& \operatorname{Bi} (x)\approx \operatorname{e} ^{\biggl(\frac{-2}{3 \xi^{3}}\biggr)} \sqrt{\xi} \sum_{n = 0}^{\infty} u (n) \xi^{n} 
\end{split} 
\end{equation*}
where $\xi = -\sqrt{\frac{1}{x}}$ The coefficients $u (n)$ satisfy the following recurrence

\begin{equation*} 
\begin{split} 
16 u (n) n + u (n - 3) \bigl(-31 + 12 n + 4 (n - 3)^{2}\bigr)& =0. 
\end{split} 
\end{equation*}
whose initial conditions are given by

\begin{equation*} 
\begin{split} 
u (2)& =0, \\ 
u (1)& =0, \\ 
u (0)& =\frac{-i}{\sqrt{\pi}}. 
\end{split} 
\end{equation*}
This recurrence has the closed form solution

\begin{equation*} 
\begin{split} 
u (3 n + 2)& =0, \\ 
u (3 n + 1)& =0, \\ 
u (3 n)& =\frac{-i(-1)^{n} 6^{(2 n)} \Gamma \Bigl(n + \frac{5}{6}\Bigr) \Gamma \Bigl(n + \frac{1}{6}\Bigr)}{2 \pi^{\frac{3}{2}} 48^{n} \Gamma (n + 1)}. 
\end{split} 
\end{equation*}
 
 
 
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