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# bessely

Bessel function of second kind

## Syntax

```Y = bessely(nu,Z) Y = bessely(nu,Z,1) ```

## Description

`Y = bessely(nu,Z)` computes Bessel functions of the second kind, Yν(z), for each element of the array `Z`. The order `nu` need not be an integer, but must be real. The argument `Z` can be complex. The result is real where `Z` is positive.

If `nu` and `Z` are arrays of the same size, the result is also that size. If either input is a scalar, it is expanded to the other input's size.

`Y = bessely(nu,Z,1)` computes `bessely(nu,Z).*exp(-abs(imag(Z)))`.

## Examples

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Create a column vector of domain values.

`z = (0:0.2:1)';`

Calculate the function values using `bessely` with `nu = 1`.

`bessely(1,z)`
```ans = 6×1 -Inf -3.3238 -1.7809 -1.2604 -0.9781 -0.7812 ```

Define the domain.

`X = 0:0.1:20;`

Calculate the first five Bessel functions of the second kind.

```Y = zeros(5,201); for i = 0:4 Y(i+1,:) = bessely(i,X); end```

Plot the results.

```plot(X,Y,'LineWidth',1.5) axis([-0.1 20.2 -2 0.6]) grid on legend('Y_0','Y_1','Y_2','Y_3','Y_4','Location','Best') title('Bessel Functions of the Second Kind for v = 0,1,2,3,4') xlabel('X') ylabel('Y_v(X)')```

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### Bessel’s Equation

The differential equation

`${z}^{2}\frac{{d}^{2}y}{d{z}^{2}}+z\frac{dy}{dz}+\left({z}^{2}-{\nu }^{2}\right)y=0,$`

where ν is a real constant, is called Bessel's equation, and its solutions are known as Bessel functions.

A solution Yν(z) of the second kind can be expressed as

`${Y}_{\nu }\left(z\right)=\frac{{J}_{\nu }\left(z\right)\mathrm{cos}\left(\nu \pi \right)-{J}_{-\nu }\left(z\right)}{\mathrm{sin}\left(\nu \pi \right)}$`

where Jν(z) and Jν(z) form a fundamental set of solutions of Bessel's equation for noninteger ν

`${J}_{v}\left(z\right)={\left(\frac{z}{2}\right)}^{\nu }\sum _{k=0}^{\infty }\frac{{\left(-\frac{{z}^{2}}{4}\right)}^{k}}{k!\Gamma \left(\nu +k+1\right)},$`

and Γ(a) is the gamma function. Yν(z) is linearly independent of Jν(z).

Jν(z) can be computed using `besselj`.

## Tips

The Bessel functions are related to the Hankel functions, also called Bessel functions of the third kind,

`$\begin{array}{l}{H}_{\nu }^{\left(1\right)}\left(z\right)={J}_{\nu }\left(z\right)+i\text{\hspace{0.17em}}{Y}_{\nu }\left(z\right)\\ {H}_{\nu }^{\left(2\right)}\left(z\right)={J}_{\nu }\left(z\right)-i\text{\hspace{0.17em}}{Y}_{\nu }\left(z\right),\end{array}$`

where ${H}_{\nu }^{\left(K\right)}\left(z\right)$ is `besselh`, Jν(z) is `besselj`, and Yν(z) is `bessely`. The Hankel functions also form a fundamental set of solutions to Bessel's equation (see `besselh`).