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# What's new in v0.3.1

```{versionadded} v0.3.1
```

v0.3.1 (2026-06-22) is a small point release. It adds one function, early,
for a downstream project that needed a differentiable linear solve from a
`pip install` before 0.4.0. Nothing else changed: no existing surface moved.

## What is new

- **`maddening.core.solver_utils.ift_linear_solve`** (`EXPERIMENTAL`): a
  differentiable linear solve. It wraps `lineax.linear_solve`, so a node that
  solves a linear system in `update()` gets gradients through the solve without
  a `custom_vjp` of its own. The solvers are `"gmres"` (the default, with its
  restart set to `min(N, 50)`), `"cg"` (for a symmetric positive-definite
  matrix) and `"dense"`. A `preconditioner` is passed on to lineax.

## Dependencies

- A new `[ift]` extra installs `lineax>=0.0.7`. `lineax` is imported only when
  `ift_linear_solve` is called, so the base install did not change.

## Its status in 0.4.0

- `ift_linear_solve` is still `EXPERIMENTAL`. 0.3.1 said it would become
  `STABLE` with `AdaptiveNode`; 0.4.0 promoted nothing to `STABLE`.
- `lineax` is a base dependency in 0.4.0, and the `[ift]` extra installs
  nothing.
- Its default `atol` is relative to the right-hand side (`MADD-ANO-123`). It
  was an absolute `1e-8`.
- 0.4.0 fixes two of its defects. With `solver="cg"` or `"gmres"`, 0.3.1 could
  return a value that was not the solution, with no error (`MADD-ANO-283`), and
  for a right-hand side with a NaN or infinite entry it returned zeros with no
  error (`MADD-ANO-155`).
- With `solver="cg"` it can still return NaN where lineax's CG breaks down
  (`MADD-ANO-264`, partly resolved). Use `solver="gmres"`.

The [0.4.0 release notes](v0.4.0.md) have the rest. The changelog's `[0.3.1]`
entry is the itemised list.