# Getting started¤

Optimistix is a JAX library for nonlinear solvers: root finding, minimisation, fixed points, and least squares.

Features include:

- interoperable solvers: e.g. autoconvert root find problems to least squares problems, then solve using a minimsation algorithm.
- modular optimisers: e.g. use a BFGS model function with a dogleg descent path with a trust region update.
- using a PyTree as the state.
- fast compilation and runtimes.
- interoperability with Optax.
- all the benefits of working with JAX: autodiff, autoparallism, GPU/TPU support etc.

## Installation¤

```
pip install optimistix
```

Requires Python 3.9+ and JAX 0.4.14+ and Equinox 0.11.0+.

## Quick example¤

```
import jax.numpy as jnp
import optimistix as optx
# Let's solve the ODE dy/dt=tanh(y(t)) with the implicit Euler method.
# We need to find y1 s.t. y1 = y0 + tanh(y1)dt.
y0 = jnp.array(1.)
dt = jnp.array(0.1)
def fn(y, args):
return y0 + jnp.tanh(y) * dt
solver = optx.Newton(rtol=1e-5, atol=1e-5)
sol = optx.fixed_point(fn, solver, y0)
y1 = sol.value # satisfies y1 == fn(y1)
```

## Next steps¤

Check out the examples or the API references on the left-hand bar.

## JAX ecosystem¤

Equinox: neural networks.

Optax: first-order gradient (SGD, Adam, ...) optimisers.

Diffrax: numerical differential equation solvers.

Lineax: linear solvers.

jaxtyping: type annotations for shape/dtype of arrays.

Eqxvision: computer vision models.

sympy2jax: SymPy<->JAX conversion; train symbolic expressions via gradient descent.

Levanter: scalable+reliable training of foundation models (e.g. LLMs).