# shrinkage::TwiceDifferentiableLink

Shrinkage for Rust · 0.3.0

[Source](<https://github.com/jolars/shrinkage/blob/eeafe449edce19d3c552a03060f681ae3891af74/src/link.rs>)

```rust
pub trait TwiceDifferentiableLink: Link
```

A link that also supplies the exact second derivative of its inverse. 

<a id="methods"></a>

## Methods

<a id="method-inverse_link_derivative_ratio"></a>

### `inverse_link_derivative_ratio`

```rust
fn inverse_link_derivative_ratio(&self, eta: f64) -> Result<f64, FitError>
```

[Source](<https://github.com/jolars/shrinkage/blob/eeafe449edce19d3c552a03060f681ae3891af74/src/link.rs>)

Evaluate `mu_second / mu_prime`. 

Override the default quotient when the derivatives underflow separately. 

<a id="method-inverse_link_derivative_ratio-documentation-errors"></a>

#### Errors

Reject invalid predictors or a nonfinite ratio. 

<a id="method-inverse_link_second_derivative"></a>

### `inverse_link_second_derivative`

```rust
fn inverse_link_second_derivative(&self, eta: f64) -> Result<f64, FitError>
```

[Source](<https://github.com/jolars/shrinkage/blob/eeafe449edce19d3c552a03060f681ae3891af74/src/link.rs>)

Evaluate `d² mu / d eta²`. 

<a id="method-inverse_link_second_derivative-documentation-errors"></a>

#### Errors

Reject invalid predictors or nonfinite arithmetic. 

<a id="impl-f96dfda96bc038ff3bbc40fbf3de973e0735a18adf6c5fbaa4477b85b174636f"></a>

## [Implementation of TwiceDifferentiableLink](<c41eda899b5b627e2c29e844d131210faac5c4524216344b1c755cf9bbe86f99>)

[Source](<https://github.com/jolars/shrinkage/blob/eeafe449edce19d3c552a03060f681ae3891af74/src/link.rs>)

```rust
impl TwiceDifferentiableLink for Identity
```

<a id="impl-bb2d2ecd7fd5b00ea455481d418b42abe21c454fae9d2450622e58288c7045bf"></a>

## [Implementation of TwiceDifferentiableLink](<c41eda899b5b627e2c29e844d131210faac5c4524216344b1c755cf9bbe86f99>)

[Source](<https://github.com/jolars/shrinkage/blob/eeafe449edce19d3c552a03060f681ae3891af74/src/link.rs>)

```rust
impl TwiceDifferentiableLink for Logit
```

<a id="impl-ca5757fa9ee28ecf54f87996652986f64c1619917ab4c0fda48a65d731d26592"></a>

## [Implementation of TwiceDifferentiableLink](<c41eda899b5b627e2c29e844d131210faac5c4524216344b1c755cf9bbe86f99>)

[Source](<https://github.com/jolars/shrinkage/blob/eeafe449edce19d3c552a03060f681ae3891af74/src/link.rs>)

```rust
impl TwiceDifferentiableLink for Probit
```