Airy function

Plot of Ai(x) in red and Bi(x) in blue

For real values of x, the Airy function of the first kind can be defined by the improper Riemann integral:

$${\displaystyle \operatorname {Ai} (x)={\dfrac {1}{\pi }}\int _{0}^{\infty }\cos \left({\dfrac {t^{3}}{3}}+xt\right)\,dt\equiv {\dfrac {1}{\pi }}\lim _{b\to \infty }\int _{0}^{b}\cos \left({\dfrac {t^{3}}{3}}+xt\right)\,dt,}$$

which converges by Dirichlet's test. For any real number x there is a positive real number M such that function ${\textstyle {\tfrac {t^{3}}{3}}+xt}$ is increasing, unbounded and convex with continuous and unbounded derivative on interval ${\displaystyle [M,\infty ).}$ The convergence of the integral on this interval can be proven by Dirichlet's test after substitution ${\textstyle u={\tfrac {t^{3}}{3}}+xt.}$

y = Ai(x) satisfies the Airy equation

$${\displaystyle y''-xy=0.}$$

This equation has two linearly independent solutions. Up to scalar multiplication, Ai(x) is the solution subject to the condition y → 0 as x → ∞. The standard choice for the other solution is the Airy function of the second kind, denoted Bi(x). It is defined as the solution with the same amplitude of oscillation as Ai(x) as x → −∞ which differs in phase by π/2:

Plot of the Airy function Bi(z) in the complex plane from -2 - 2i to 2 + 2i with colors created with Mathematica 13.1 function ComplexPlot3D

$${\displaystyle \operatorname {Bi} (x)={\frac {1}{\pi }}\int _{0}^{\infty }\left[\exp \left(-{\tfrac {t^{3}}{3}}+xt\right)+\sin \left({\tfrac {t^{3}}{3}}+xt\right)\,\right]dt.}$$

Plot of the derivative of the Airy function Bi'(z) in the complex plane from -2 - 2i to 2 + 2i with colors created with Mathematica 13.1 function ComplexPlot3D

The values of Ai(x) and Bi(x) and their derivatives at x = 0 are given by

$${\displaystyle {\begin{aligned}\operatorname {Ai} (0)&{}={\frac {1}{3^{2/3}\,\Gamma \!\left({\frac {2}{3}}\right)}},&\quad \operatorname {Ai} '(0)&{}=-{\frac {1}{3^{1/3}\,\Gamma \!\left({\frac {1}{3}}\right)}},\\\operatorname {Bi} (0)&{}={\frac {1}{3^{1/6}\,\Gamma \!\left({\frac {2}{3}}\right)}},&\quad \operatorname {Bi} '(0)&{}={\frac {3^{1/6}}{\Gamma \!\left({\frac {1}{3}}\right)}}.\end{aligned}}}$$

Here, Γ denotes the Gamma function. It follows that the Wronskian of Ai(x) and Bi(x) is 1/π.

When x is positive, Ai(x) is positive, convex, and decreasing exponentially to zero, while Bi(x) is positive, convex, and increasing exponentially. When x is negative, Ai(x) and Bi(x) oscillate around zero with ever-increasing frequency and ever-decreasing amplitude. This is supported by the asymptotic formulae below for the Airy functions.

The Airy functions are orthogonal[1] in the sense that

$${\displaystyle \int _{-\infty }^{\infty }\operatorname {Ai} (t+x)\operatorname {Ai} (t+y)dt=\delta (x-y)}$$

again using an improper Riemann integral.

Real zeros of Ai(x) and its derivative Ai'(x)

Neither Ai(x) nor its derivative Ai(x) have positive real zeros. The "first" real zeros (i.e. nearest to x=0) are:[2]

• "first" zeros of Ai(x) are at x ≈ −2.33811, −4.08795, −5.52056, −6.78671, ...
• "first" zeros of its derivative Ai'(x) are at x ≈ −1.01879, −3.24820, −4.82010, −6.16331, ...

Ai(blue) and sinusoidal/exponential asymptotic form of Ai(magenta)

Bi(blue) and sinusoidal/exponential asymptotic form of Bi(magenta)

As explained below, the Airy functions can be extended to the complex plane, giving entire functions. The asymptotic behaviour of the Airy functions as |z| goes to infinity at a constant value of arg(z) depends on arg(z): this is called the Stokes phenomenon. For |arg(z)| < π we have the following asymptotic formula for Ai(z):[3]

$${\displaystyle \operatorname {Ai} (z)\sim {\dfrac {1}{2{\sqrt {\pi }}\,z^{1/4}}}\exp \left(-{\frac {2}{3}}z^{3/2}\right)\left[\sum _{n=0}^{\infty }{\dfrac {(-1)^{n}\,\Gamma \!\left(n+{\frac {5}{6}}\right)\,\Gamma \!\left(n+{\frac {1}{6}}\right)\left({\frac {3}{4}}\right)^{n}}{2\pi \,n!\,z^{3n/2}}}\right].}$$

or

$${\displaystyle \operatorname {Ai} (z)\sim {\dfrac {e^{-\zeta }}{4\pi ^{3/2}\,z^{1/4}}}\left[\sum _{n=0}^{\infty }{\dfrac {\Gamma \!\left(n+{\frac {5}{6}}\right)\,\Gamma \!\left(n+{\frac {1}{6}}\right)}{n!(-2\zeta )^{n}}}\right].}$$

where ${\displaystyle \zeta ={\tfrac {2}{3}}z^{3/2}.}$ In particular, the first few terms are[4]

$${\displaystyle \operatorname {Ai} (z)={\frac {e^{-\zeta }}{2\pi ^{1/2}z^{1/4}}}\left(1-{\frac {5}{72\zeta }}+{\frac {385}{10368\zeta ^{2}}}+O(\zeta ^{-3})\right)}$$

There is a similar one for Bi(z), but only applicable when |arg(z)| < π/3:

$${\displaystyle \operatorname {Bi} (z)\sim {\frac {1}{{\sqrt {\pi }}\,z^{1/4}}}\exp \left({\frac {2}{3}}z^{3/2}\right)\left[\sum _{n=0}^{\infty }{\dfrac {\Gamma \!\left(n+{\frac {5}{6}}\right)\,\Gamma \!\left(n+{\frac {1}{6}}\right)\left({\frac {3}{4}}\right)^{n}}{2\pi \,n!\,z^{3n/2}}}\right].}$$

A more accurate formula for Ai(z) and a formula for Bi(z) when π/3 < |arg(z)| < π or, equivalently, for Ai(−z) and Bi(−z) when |arg(z)| < 2π/3 but not zero, are:[3][5]

$${\displaystyle {\begin{aligned}\operatorname {Ai} (-z)\sim &{}\ {\frac {1}{{\sqrt {\pi }}\,z^{1/4}}}\sin \left({\frac {2}{3}}z^{3/2}+{\frac {\pi }{4}}\right)\left[\sum _{n=0}^{\infty }{\dfrac {(-1)^{n}\,\Gamma \!\left(2n+{\frac {5}{6}}\right)\,\Gamma \!\left(2n+{\frac {1}{6}}\right)\left({\frac {3}{4}}\right)^{2n}}{2\pi \,(2n)!\,z^{3n}}}\right]\\[6pt]&{}-{\frac {1}{{\sqrt {\pi }}\,z^{1/4}}}\cos \left({\frac {2}{3}}z^{3/2}+{\frac {\pi }{4}}\right)\left[\sum _{n=0}^{\infty }{\dfrac {(-1)^{n}\,\Gamma \!\left(2n+{\frac {11}{6}}\right)\,\Gamma \!\left(2n+{\frac {7}{6}}\right)\left({\frac {3}{4}}\right)^{2n+1}}{2\pi \,(2n+1)!\,z^{3n\,+\,3/2}}}\right]\\[6pt]\operatorname {Bi} (-z)\sim &{}{\frac {1}{{\sqrt {\pi }}\,z^{1/4}}}\cos \left({\frac {2}{3}}z^{3/2}+{\frac {\pi }{4}}\right)\left[\sum _{n=0}^{\infty }{\dfrac {(-1)^{n}\,\Gamma \!\left(2n+{\frac {5}{6}}\right)\,\Gamma \!\left(2n+{\frac {1}{6}}\right)\left({\frac {3}{4}}\right)^{2n}}{2\pi \,(2n)!\,z^{3n}}}\right]\\[6pt]&{}+{\frac {1}{{\sqrt {\pi }}\,z^{\frac {1}{4}}}}\sin \left({\frac {2}{3}}z^{3/2}+{\frac {\pi }{4}}\right)\left[\sum _{n=0}^{\infty }{\dfrac {(-1)^{n}\,\Gamma \!\left(2n+{\frac {11}{6}}\right)\,\Gamma \!\left(2n+{\frac {7}{6}}\right)\left({\frac {3}{4}}\right)^{2n+1}}{2\pi \,(2n+1)!\,z^{3n\,+\,3/2}}}\right].\end{aligned}}}$$

When |arg(z)| = 0 these are good approximations but are not asymptotic because the ratio between Ai(−z) or Bi(−z) and the above approximation goes to infinity whenever the sine or cosine goes to zero. Asymptotic expansions for these limits are also available. These are listed in (Abramowitz and Stegun, 1983) and (Olver, 1974).

One is also able to obtain asymptotic expressions for the derivatives Ai'(z) and Bi'(z). Similarly to before, when |arg(z)| < π:[5]

$${\displaystyle \operatorname {Ai} '(z)\sim -{\dfrac {z^{1/4}}{2{\sqrt {\pi }}\,}}\exp \left(-{\frac {2}{3}}z^{3/2}\right)\left[\sum _{n=0}^{\infty }{\frac {1+6n}{1-6n}}{\dfrac {(-1)^{n}\,\Gamma \!\left(n+{\frac {5}{6}}\right)\,\Gamma \!\left(n+{\frac {1}{6}}\right)\left({\frac {3}{4}}\right)^{n}}{2\pi \,n!\,z^{3n/2}}}\right].}$$

When |arg(z)| < π/3 we have:[5]

$${\displaystyle \operatorname {Bi} '(z)\sim {\frac {z^{1/4}}{{\sqrt {\pi }}\,}}\exp \left({\frac {2}{3}}z^{3/2}\right)\left[\sum _{n=0}^{\infty }{\frac {1+6n}{1-6n}}{\dfrac {\Gamma \!\left(n+{\frac {5}{6}}\right)\,\Gamma \!\left(n+{\frac {1}{6}}\right)\left({\frac {3}{4}}\right)^{n}}{2\pi \,n!\,z^{3n/2}}}\right].}$$

Similarly, an expression for Ai'(−z) and Bi'(−z) when |arg(z)| < 2π/3 but not zero, are[5]

$${\displaystyle {\begin{aligned}\operatorname {Ai} '(-z)\sim &{}-{\frac {z^{1/4}}{{\sqrt {\pi }}\,}}\cos \left({\frac {2}{3}}z^{3/2}+{\frac {\pi }{4}}\right)\left[\sum _{n=0}^{\infty }{\frac {1+12n}{1-12n}}{\dfrac {(-1)^{n}\,\Gamma \!\left(2n+{\frac {5}{6}}\right)\,\Gamma \!\left(2n+{\frac {1}{6}}\right)\left({\frac {3}{4}}\right)^{2n}}{2\pi \,(2n)!\,z^{3n}}}\right]\\[6pt]&{}-{\frac {z^{1/4}}{{\sqrt {\pi }}\,}}\sin \left({\frac {2}{3}}z^{3/2}+{\frac {\pi }{4}}\right)\left[\sum _{n=0}^{\infty }{\frac {7+12n}{-5-12n}}{\dfrac {(-1)^{n}\,\Gamma \!\left(2n+{\frac {11}{6}}\right)\,\Gamma \!\left(2n+{\frac {7}{6}}\right)\left({\frac {3}{4}}\right)^{2n+1}}{2\pi \,(2n+1)!\,z^{3n\,+\,3/2}}}\right]\\[6pt]\operatorname {Bi} '(-z)\sim &{}\ {\frac {z^{1/4}}{{\sqrt {\pi }}\,}}\sin \left({\frac {2}{3}}z^{3/2}+{\frac {\pi }{4}}\right)\left[\sum _{n=0}^{\infty }{\frac {1+12n}{1-12n}}{\dfrac {(-1)^{n}\,\Gamma \!\left(2n+{\frac {5}{6}}\right)\,\Gamma \!\left(2n+{\frac {1}{6}}\right)\left({\frac {3}{4}}\right)^{2n}}{2\pi \,(2n)!\,z^{3n}}}\right]\\[6pt]&{}-{\frac {z^{1/4}}{{\sqrt {\pi }}\,}}\cos \left({\frac {2}{3}}z^{3/2}+{\frac {\pi }{4}}\right)\left[\sum _{n=0}^{\infty }{\frac {7+12n}{-5-12n}}{\dfrac {(-1)^{n}\,\Gamma \!\left(2n+{\frac {11}{6}}\right)\,\Gamma \!\left(2n+{\frac {7}{6}}\right)\left({\frac {3}{4}}\right)^{2n+1}}{2\pi \,(2n+1)!\,z^{3n\,+\,3/2}}}\right]\\\end{aligned}}}$$

We can extend the definition of the Airy function to the complex plane by

$${\displaystyle \operatorname {Ai} (z)={\frac {1}{2\pi i}}\int _{C}\exp \left({\tfrac {t^{3}}{3}}-zt\right)\,dt,}$$

where the integral is over a path C starting at the point at infinity with argument −π/3 and ending at the point at infinity with argument π/3. Alternatively, we can use the differential equation y′′ − xy = 0 to extend Ai(x) and Bi(x) to entire functions on the complex plane.

The asymptotic formula for Ai(x) is still valid in the complex plane if the principal value of x^2/3 is taken and x is bounded away from the negative real axis. The formula for Bi(x) is valid provided x is in the sector ${\displaystyle x\in \mathbb {C}$ :\left|\arg(x)\right|<{\tfrac {\pi }{3}}-\delta } for some positive δ. Finally, the formulae for Ai(−x) and Bi(−x) are valid if x is in the sector ${\displaystyle x\in \mathbb {C}$ :\left|\arg(x)\right|<{\tfrac {2\pi }{3}}-\delta .}

It follows from the asymptotic behaviour of the Airy functions that both Ai(x) and Bi(x) have an infinity of zeros on the negative real axis. The function Ai(x) has no other zeros in the complex plane, while the function Bi(x) also has infinitely many zeros in the sector ${\displaystyle z\in \mathbb {C}$ :{\tfrac {\pi }{3}}<\left|\arg(z)\right|<{\tfrac {\pi }{2}}.}

Plots

${\displaystyle \Re \left[\operatorname {Ai} (x+iy)\right]}$	${\displaystyle \Im \left[\operatorname {Ai} (x+iy)\right]}$	${\displaystyle \left|\operatorname {Ai} (x+iy)\right|\,}$	${\displaystyle \operatorname {arg} \left[\operatorname {Ai} (x+iy)\right]\,}$

${\displaystyle \Re \left[\operatorname {Bi} (x+iy)\right]}$	${\displaystyle \Im \left[\operatorname {Bi} (x+iy)\right]}$	${\displaystyle \left|\operatorname {Bi} (x+iy)\right|\,}$	${\displaystyle \operatorname {arg} \left[\operatorname {Bi} (x+iy)\right]\,}$

For positive arguments, the Airy functions are related to the modified Bessel functions:

$${\displaystyle {\begin{aligned}\operatorname {Ai} (x)&{}={\frac {1}{\pi }}{\sqrt {\frac {x}{3}}}\,K_{1/3}\!\left({\frac {2}{3}}x^{3/2}\right),\\\operatorname {Bi} (x)&{}={\sqrt {\frac {x}{3}}}\left[I_{1/3}\!\left({\frac {2}{3}}x^{3/2}\right)+I_{-1/3}\!\left({\frac {2}{3}}x^{3/2}\right)\right].\end{aligned}}}$$

Here, I_±1/3 and K_1/3 are solutions of

$${\displaystyle x^{2}y''+xy'-\left(x^{2}+{\tfrac {1}{9}}\right)y=0.}$$

The first derivative of the Airy function is

$${\displaystyle \operatorname {Ai'} (x)=-{\frac {x}{\pi {\sqrt {3}}}}\,K_{2/3}\!\left({\frac {2}{3}}x^{3/2}\right).}$$

Functions K_1/3 and K_2/3 can be represented in terms of rapidly convergent integrals[6] (see also modified Bessel functions)

For negative arguments, the Airy function are related to the Bessel functions:

$${\displaystyle {\begin{aligned}\operatorname {Ai} (-x)&{}={\sqrt {\frac {x}{9}}}\left[J_{1/3}\!\left({\frac {2}{3}}x^{3/2}\right)+J_{-1/3}\!\left({\frac {2}{3}}x^{3/2}\right)\right],\\\operatorname {Bi} (-x)&{}={\sqrt {\frac {x}{3}}}\left[J_{-1/3}\!\left({\frac {2}{3}}x^{3/2}\right)-J_{1/3}\!\left({\frac {2}{3}}x^{3/2}\right)\right].\end{aligned}}}$$

Here, J_±1/3 are solutions of

$${\displaystyle x^{2}y''+xy'+\left(x^{2}-{\frac {1}{9}}\right)y=0.}$$

The Scorer's functions Hi(x) and -Gi(x) solve the equation y′′ − xy = 1/π. They can also be expressed in terms of the Airy functions:

$${\displaystyle {\begin{aligned}\operatorname {Gi} (x)&{}=\operatorname {Bi} (x)\int _{x}^{\infty }\operatorname {Ai} (t)\,dt+\operatorname {Ai} (x)\int _{0}^{x}\operatorname {Bi} (t)\,dt,\\\operatorname {Hi} (x)&{}=\operatorname {Bi} (x)\int _{-\infty }^{x}\operatorname {Ai} (t)\,dt-\operatorname {Ai} (x)\int _{-\infty }^{x}\operatorname {Bi} (t)\,dt.\end{aligned}}}$$

Using the definition of the Airy function Ai(x), it is straightforward to show its Fourier transform is given by

$${\displaystyle {\mathcal {F}}(\operatorname {Ai} )(k):=\int _{-\infty }^{\infty }\operatorname {Ai} (x)\ e^{-2\pi ikx}\,dx=e^{{\frac {i}{3}}(2\pi k)^{3}}.}$$

This can be obtained by taking the Fourier transform of the Airy equation. Let ${\textstyle {\hat {y}}={\frac {1}{2\pi i}}\int ye^{-ikx}dx}$, then ${\displaystyle i{\hat {y}}'+k^{2}{\hat {y}}=0}$, which then has solutions ${\displaystyle {\hat {y}}=Ce^{ik^{3}}.}$ Why is there only one dimension of solutions? Because the Fourier transform requires y to decay to zero fast enough, and unfortunately Bi(+∞) = +∞ exponentially fast, so it cannot be found by Fourier transform.

The Airy function is named after the British astronomer and physicist George Biddell Airy (1801–1892), who encountered it in his early study of optics in physics (Airy 1838). The notation Ai(x) was introduced by Harold Jeffreys. Airy had become the British Astronomer Royal in 1835, and he held that post until his retirement in 1881.

• The proof of Witten's conjecture used a matrix-valued generalization of the Airy function.
• Airy zeta function

• Abramowitz, Milton; Stegun, Irene Ann, eds. (1983) [June 1964]. "Chapter 10". Handbook of Mathematical Functions with Formulas, Graphs, and Mathematical Tables. Applied Mathematics Series. Vol. 55 (Ninth reprint with additional corrections of tenth original printing with corrections (December 1972); first ed.). Washington D.C.; New York: United States Department of Commerce, National Bureau of Standards; Dover Publications. p. 448. ISBN 978-0-486-61272-0. LCCN 64-60036. MR 0167642. LCCN 65-12253.
• Airy (1838), "On the intensity of light in the neighbourhood of a caustic", Transactions of the Cambridge Philosophical Society, University Press, 6: 379–402, Bibcode:1838TCaPS...6..379A
• Frank William John Olver (1974). Asymptotics and Special Functions, Chapter 11. Academic Press, New York.
• Press, WH; Teukolsky, SA; Vetterling, WT; Flannery, BP (2007), "Section 6.6.3. Airy Functions", Numerical Recipes: The Art of Scientific Computing (3rd ed.), New York: Cambridge University Press, ISBN 978-0-521-88068-8, archived from the original on 2011-08-11, retrieved 2011-08-09
• Vallée, Olivier; Soares, Manuel (2004), Airy functions and applications to physics, London: Imperial College Press, ISBN 978-1-86094-478-9, MR 2114198, archived from the original on 2010-01-13, retrieved 2010-05-14

• "Airy functions", Encyclopedia of Mathematics, EMS Press, 2001 [1994]
• Weisstein, Eric W. "Airy Functions". MathWorld.
• Wolfram function pages for Ai and Bi functions. Includes formulas, function evaluator, and plotting calculator.
• Olver, F. W. J. (2010), "Airy and related functions", in Olver, Frank W. J.; Lozier, Daniel M.; Boisvert, Ronald F.; Clark, Charles W. (eds.), NIST Handbook of Mathematical Functions, Cambridge University Press, ISBN 978-0-521-19225-5, MR 2723248.