# Paths¤

Many objects that arise when solving differential equations are *paths*. That is to say, they are some piecewise continuous function \(f : [t_0, t_1] \to \mathbb{R}^d\).

Diffrax represents this general notion via the `diffrax.AbstractPath`

class. For example, Brownian motion is a subclass of this. So are the interpolations used to drive neural controlled differential equations.

If you need to create your own path (e.g. to drive a CDE) then you can do so by subclassing `diffrax.AbstractPath`

.

`diffrax.AbstractPath`

####
```
diffrax.AbstractPath
```

¤

Abstract base class for all paths.

Every path has a start point `t0`

and an end point `t1`

. In between these values
it is possible to `evaluate`

the path, or (if it is differentiable, e.g. not
Brownian motion) calculate its `derivative`

.

Example

```
class QuadraticPath(AbstractPath):
@property
def t0(self):
return 0
@property
def t1(self):
return 3
def evaluate(self, t0, t1=None, left=True):
del left
if t1 is not None:
return self.evaluate(t1) - self.evaluate(t0)
return t0 ** 2
```

#####
`evaluate(self, t0: Union[float, int], t1: Union[float, int] = None, left: bool = True) -> PyTree[Array]`

`abstractmethod`

¤

Evaluate the path at any point in the interval \([t_0, t_1]\).

**Arguments:**

`t0`

: Any point in \([t_0, t_1]\) to evaluate the path at.`t1`

: If passed, then the increment from`t1`

to`t0`

is evaluated instead.`left`

: Across jump points: whether to treat the path as left-continuous or right-continuous.

FAQ

Note that we use \(t_0\) and \(t_1\) to refer to the overall interval, as
obtained via `instance.t0`

and `instance.t1`

. We use `t0`

and `t1`

to refer
to some subinterval of \([t_0, t_1]\). This is an API that is used for
consistency with the rest of the package, and just happens to be a little
confusing here.

**Returns:**

If `t1`

is not passed:

The value of the path at `t0`

.

If `t1`

is passed:

The increment of the path between `t0`

and `t1`

.

#####
`derivative(self, t: Union[float, int], left: bool = True) -> PyTree[Array]`

¤

Evaluate the derivative of the path. Essentially equivalent
to `jax.jvp(self.evaluate, (t,), (jnp.ones_like(t),))`

(and indeed this is its
default implementation if no other is specified).

**Arguments:**

`t`

: Any point in \([t_0, t_1]\) to evaluate the derivative at.`left`

: Whether to obtain the left-derivative or right-derivative at that point.

**Returns:**

The derivative of the path.