# ScalarRadau

Solve a *scalar* differential equation with accuracy, efficiency, and stability

```
using BenchmarkTools, ScalarRadau, Plots
F(x, y, p) = 50*(cos(x) - y);
x, y = radau(F, 0.1, 0, 3, 1000);
plot(x, y, legend=false, xlabel="x", ylabel="y");
```

```
x = LinRange(0, 3, 100)
y = zeros(length(x))
@btime radau!($y, $x, $F, 0.1, 0.0, 3.0);
5.667 μs (2 allocations: 1.75 KiB)
```

This module contains a lightweight implementation of the classic 5th order Radau IIA method for a **scalar** ordinary differential equation (ODE) in Julia. The algorithm is famously effective when the stability of the ODE solving method is a priority. It is "stiffly accurate" and A-B-L stable. Implementation mostly follows the description in chapter IV.8 in Solving Ordinary Differential Equations II, by Ernst Hairer and Gerhard Wanner.

Some basic points of description:

- Step size is adaptive and the initial step size is chosen automatically.
- Functions implemented here mostly type-flexible. The dependent variable (
`y₀`

,`yout`

) is restricted to`<:Real`

. Also, your ODE should be type stable and probably should not return anything that isn't`<:Real`

. - Dense output for continuous solutions is implemented using cubic Hermite interpolation.
- Approximate Jacobian evaluation is performed with a simple finite difference, which costs one function evaluation for each attempted step.
- Because the equation is scalar and the 5th order Radau method has three stages, the Jacobian is always a 3 x 3 matrix. Static arrays are used for efficient quasi-Newton iterations.

The implementation here is designed for a scenario where a scalar ODE must be solved repeatedly and the stability of the solver is really important. The module was originally written to solve the Schwarzschild equation for radiative transfer as part of ClearSky.jl. In this case, the stability properties of the solver are crucial because they prevent non-physical "overshoots."

The solver functions specialize directly on the ODE provided. This is slightly different than DifferentialEquations.jl, which uses a two-step system of defining an ODE problem with one function then solving it with another function, but if you need to solve a stiff system of ODEs instead of a scalar equation, look here. Specifically, the vector implementation of the same Radau method is called `RadauIIA5`

.

For a nice mathematical overview of Radau methods, check out: Stiff differential equations solved by Radau methods.

`ScalarRadau`

How to Use Install using Julia's package manager

`julia> ]add ScalarRadau`

To solve an ODE, first define it as a function, then pass it to the `radau`

function.

```
using ScalarRadau
F(x, y, param) = -y
x, y = radau(F, 1.0, 0, 2, 25)
```

The snippet above solves the equation `dy/dx = -y`

, starting at `y=1`

, between `x=0`

and `x=2`

, and returns 25 evenly spaced points in the solution interval.

The "function" `F`

can be any callable object, as long as it can be called with `(x, y, param)`

arguments.

### In-place Solution

For full control over output points, the in-place function is

`radau!(yout, xout, F, y₀, x₀, xₙ, param=nothing; rtol=1e-6, atol=1e-6, facmax=100.0, facmin=0.01, κ=1e-3, ϵ=0.25, maxnewt=7, maxstep=1000000, maxfail=10)`

Mandatory function arguments are

`yout`

- vector where output points will be written`xout`

- sorted vector of`x`

values where output points should be sampled`F`

- scalar ODE in the form`dy/dx = F(x, y, param)`

`y₀`

- initial value for`y`

`x₀`

- starting point for`x`

`xₙ`

- end point of the integration

The optional `param`

argument is `nothing`

by default, but it may be any type and is meant for scenarios where extra information must be accessible to the ODE function. It is passed to `F`

whenever it's called.

The coordinates of the output points (`xout`

) should be between `x₀`

and `xₙ`

and they should be in ascending order. They are not checked for integrity before integrating. The only check performed is `xₙ > x₀`

, or that the integration isn't going backward.

**When solving in-place, values in yout are added to, not overwritten.** This means that if

`yout`

is full of NaNs or any other non-zero values upon calling `radau!`

, they will be present in the result. You must pre-fill `yout`

with zeros on your own.Keyword arguments are

`rtol`

- relative error tolerance`atol`

- absolute error tolerance`facmax`

- maximum fraction that the step size may increase, compared to the previous step`facmin`

- minimum fraction that the step size may decrease, compared to the previous step`κ`

(kappa) - stopping tolerance for Newton iterations`ϵ`

(epsilon) - fraction of current step size used for finite difference Jacobian approximation`maxnewt`

- maximum number of Newton iterations before step size reduction`maxstep`

- maximum number of steps before the solver stops and throws an error`maxfail`

- maximum number of Newton convergence failures before error

The `maxnewt`

, `maxstep`

, and `maxfail`

arguments are not restricted to integers, so they can be set to `Inf`

to effectively disable them.

Two other functions, described below, are available to make different output options convenient. Both of them use the in-place version internally.

### Evenly Spaced Output

For evenly spaced output points (as in the example above) the function definition is

`radau(F, y₀, x₀, xₙ, nout, param=nothing; kwargs...)`

In this case, you must specify the number of output points with the `nout`

argument. Keyword arguments and default values are the same as above. Solution vectors for `x`

and `y`

are returned.

### End-point Output

To compute only the `y`

value at the end of the integration interval (`xₙ`

), the function is

`radau(F, y₀, x₀, xₙ, param=nothing; kwargs...)`

Again, keyword arguments and default values are identical to the in-place function.