# Bessel Function Calculator

- Bessel functions and its differential equation
- How do you calculate the Bessel function of the first kind?
- How do you calculate the Bessel function of the second kind?
- How do you calculate Hankel functions?
- Recurrence relation of Bessel functions
- How to find Bessel function values using this Bessel function calculator
- FAQ

Are you struggling to calculate or validate **Bessel function values**? Do you wish you could **plot** the Bessel function to get that extra information? If your answer is yes, you've come to the right place because our **Bessel function calculator** does everything for you!

Bessel functions are fairly advanced mathematical topics that can be perplexing to anyone. This article covers the basics, such as the **Bessel differential equation**, how to **calculate Bessel functions** of the **first** and **second kinds**, and the recurrence relations for Bessel functions, so you're well equipped to solve your problem using Bessel functions.

## Bessel functions and its differential equation

**Bessel differential** equation is a *second-order* differential equation given by:

Here $\nu$ is an arbitrary complex number.

Since this is a second-order differential equation, there have to be **two linearly independent solutions**. We call these solutions **Bessel functions** of the first and second kind. All Bessel functions are also commonly referred to as **cylinder functions**.

The **order** of the Bessel function is given by $\nu$, and although it can be an arbitrary complex number, the most critical cases are when $\nu$ is an *integer* or *half-integer*. In this calculator, we shall exclusively use **real-valued** $\nu$, but $x$ can be complex.

You can refresh your knowledge of complex number operations using our complex number calculator.

## How do you calculate the Bessel function of the first kind?

We use the following **power-series** to evaluate the Bessel function of the first kind:

where:

- $J_\nu(x)$ –
**Bessel function of the first kind**; - $\nu$ –
**Order**of the Bessel function; - $x$ – Arbitrary
**real or complex**number; and - $\Gamma(z)$ –
**Gamma function**, an extension of the**factorial function**to*non-integer values*.

You can learn more about this $\Gamma(z)$ using our gamma function calculator.

For *non-integer* $\nu$, the functions $J_\nu(x)$ and $J_{-\nu}(x)$ are linearly independent. However, for *integer* $\nu$, they're related as follows:

💡 For computation, it is impractical to expand this series to $\infin$. We should get sufficiently precise values after a **finite** number of iterations.

## How do you calculate the Bessel function of the second kind?

Computing the Bessel function of the second kind is tricker than $J_\nu(x)$ because it has different formulae for different $\nu$.

To evaluate the Bessel function of the second kind for * non-integer* $\nu$, we use the formula:

where $Y_\nu(x)$ is the **Bessel function of the second kind** of order $\nu$.

If the order $\nu$ is an * integer* $n$, we should take the limit of $Y_\nu(x)$ as the order $\nu$ approaches the integer $n$:

For * non-negative integer* $n$, this limit reduces to:

where $\psi(z)$ is the **digamma function**, the logarithmic derivative of the gamma function $\Gamma(z)$:

If you're groaning at this complexity, we have some good news! Since we're using $\psi(n)$ exclusively for * non-negative* $n$, we can use a simpler formulation for the digamma function, given by:

where:

- $H_{n-1}$ – $(n-1)^{th}$
**harmonic number**; and - $\gamma$ –
**.**

But what about * negative integer* $n$? Again, we have some good news! Similar to $J_\nu(x)$, $Y_\nu(x)$ also follows the relation:

which we can utilize to calculate $Y_{-n}(x)$ from $Y_{n}$.

## How do you calculate Hankel functions?

The **Bessel functions of the third kind**, also known as the **Hankel functions**, are two linearly independent solutions to the Bessel differential equation. We express them as linear combinations of the first two kinds of Bessel functions:

where:

- $H_\nu^{(1)}(x) , H_\nu^{(2)}(x)$ – The
**Hankel functions**; and - $i$ – The
**imaginary unit**.

## Recurrence relation of Bessel functions

The Bessel functions we discussed so far exhibit the following **recurrence relations**:

where:

- $C_\nu(z)$ – Any
**cylinder function**, i.e., $J_\nu(z)$ or $Y_\nu(z)$; - $C\rq_\nu(z)$ –
**Derivative**of any cylinder function; and - $z$ – Any arbitrary
**real or complex**number.

In particular:

You can use these recurrence relations for Bessel functions to calculate the derivatives of the desired Bessel function quickly.

## How to find Bessel function values using this Bessel function calculator

This Bessel function calculator will solve for Bessel functions of the first, second, and third kind simultaneously. All you need to input are the **order** $\nu$ and $x$, the point at which you desire to evaluate. Keep in mind that:

- The
**order**$\nu$ must be a**real number**; - The number $x$ can be
*real or complex*; and - This Bessel function calculator will
**plot**the Bessel function of the first two kinds, as long as the number $x$ is a**real number**.

Note that the **order** $\nu$ must be within the range $[-99, 99]$ to keep the **computational time** to a *minimum*. Any higher order will cause noticeable lag in most computers.

Also, the $\Re(x)$ must be within the range $[-20, 20]$ to maintain **computational accuracy** since there is a *different method* for calculating Bessel functions for large $x$.

If you wish to perform calculations beyond these limits, please get in touch with us!

## FAQ

### How do I calculate bandwidth with Bessel function table?

To estimate **bandwidth** using a Bessel function table, you must know the **modulating index** `β`

and **modulating frequency** `fₘ`

:

**Find**the**minimum value**of`Jᵥ(β)`

above`0.01`

(or any value deemed the minimum significant value) by referring to a**Bessel function table**.**Determine**the**number of sideband pairs**`N`

in the signal, equal to the order`v`

of the Bessel function`Jᵥ(β)`

.**Substitute**`N`

and`fₘ`

in the formula`B = 2fₘN`

to get the**bandwidth**`B`

.- Be proud of yourself for solving a not-so-simple problem in a not-so-complex manner!

### What is the maximum value of the Bessel function of the first kind?

`J₀(0) = 1`

is the **maximum** value of the Bessel function of the first kind. It occurs when the **order** `ν = 0`

and `x = 0`

. To calculate this, follow these steps:

**Substitute**`ν = 0`

and`x = 0`

into the integrand`cos(ντ - x sin(τ))`

to get`cos(0) = 1`

.**Evaluate**the integral`∫1 ∙ dτ`

to get`[τ]`

.**Apply**the*lower limit*`0`

and the*upper limit*`π`

to`[τ]`

to get`[π-0] = π`

.**Divide**by`π`

to get`π/π = 1`

.- Verify this result using Omni's Bessel function calculator!

### Where do the singularities lie for the Bessel functions?

For * negative non-integer* orders, the

**Bessel function**of the

**first**kind has a singularity at

`x = 0`

. For all orders, the Bessel function of the second kind has a singularity at `x = 0`

.### Are Bessel functions periodic?

The Bessel functions are **not periodic**, although they look like decaying sine or cosine waves on a graph.