# Manual

IDESolver implements an iterative algorithm from this paper for solving general IDEs. The algorithm requires an ODE integrator and a quadrature integrator internally. IDESolver uses scipy.integrate.solve_ivp() as the ODE integrator. The quadrature integrator is either scipy.integrate.quad() or complex_quad(), a thin wrapper over scipy.integrate.quad() which handles splitting the real and imaginary parts of the integral.

## The Algorithm

We want to find an approximate solution to

$\begin{split}\frac{dy}{dx} & = c(y, x) + d(x) \int_{\alpha(x)}^{\beta(x)} k(x, s) \, F( y(s) ) \, ds, \\ & x \in [a, b], \quad y(a) = y_0.\end{split}$

The algorithm begins by creating an initial guess for $$y$$ by using an ODE solver on

$\frac{dy}{dx} = c(y, x)$

Since there’s no integral on the right-hand-side, standard ODE solvers can handle it easily. Call this guess $$y^{(0)}$$. We can then produce a better guess by seeing what we would get with the original IDE, but replacing $$y$$ on the right-hand-side by $$y^{(0)}$$:

$\frac{dy^{(1/2)}}{dx} = c(y^{(0)}, x) + d(x) \int_{\alpha(x)}^{\beta(x)} k(x, s) \, F( y^{(0)}(s) ) \, ds$

Again, this is just an ODE, because $$y^{(1/2)}$$ does not appear on the right. At this point in the algorithm we check the global error between $$y^{(0)}$$ and $$y^{(1/2)}$$. If it’s smaller than the tolerance, we stop iterating and take $$y^{(1/2)}$$ to be the solution. If it’s larger than the tolerance, the iteration continues. To be conservative and to make sure we don’t over-correct, we’ll combine $$y^{(1/2)}$$ with $$y^{(0)}$$.

$y^{(1)} = \alpha y^{(0)} + (1 - \alpha) y^{(1/2)}$

The process then repeats: solve the IDE-turned-ODE with $$y^{(1)}$$ on the right-hand-side, see how different it is, maybe make a new guess, etc.

## Stopping Conditions

IDESolver can operate in three modes: either a nonzero global error tolerance should be given, or a maximum number of iterations should be given, or both should be given. Nonzero global error tolerance is the standard mode, described in The Algorithm. If a maximum number of iterations is given with zero global error tolerance, the algorithm will iterate that many times and then stop. If both are given, the algorithm terminates if either condition is met.

## Global Error Estimate

The default global error estimate $$G$$ between two possible solutions $$y_1$$ and $$y_2$$ is

$G = \sqrt{ \sum_{x_i} \left| y_1(x_i) - y_2(x_i) \right| }$

A different global error estimator can be passed in the constructor as the argument global_error_function.

## Test Suite

First, get the entire IDESolver repository via git clone https://github.com/JoshKarpel/idesolver.git. Running the test suite requires some additional Python packages: run pip install -r requirements-dev.txt from the repository root to install them. Once installed, you can run the test suite by running pytest from the repository root.