# tensorplay.autograd.functional.hvp

Source: https://www.tensorplay.cn/docs/generated/tensorplay.autograd.functional.hvp.html

```python
tensorplay.autograd.functional.hvp(func, inputs, v=None, create_graph=False, strict=False)
```

Compute the dot product between the scalar function’s Hessian and a vector v at a specified point.

Parameters:

- func (function) – a Python function that takes Tensor inputs and returns a Tensor with a single element.

- inputs ([tuple](https://docs.python.org/3/builtins/stdtypes.html#tuple) of Tensors or [Tensor](/docs/generated/tensorplay.Tensor.html#tensorplay.Tensor)) – inputs to the function func.

- v ([tuple](https://docs.python.org/3/builtins/stdtypes.html#tuple) of Tensors or [Tensor](/docs/generated/tensorplay.Tensor.html#tensorplay.Tensor)) – The vector for which the Hessian vector product is computed. Must be the same size as the input of func. This argument is optional when func’s input contains a single element and (if it is not provided) will be set as a Tensor containing a single 1.

- create_graph ([bool](https://docs.python.org/3/builtins/functions.html#bool), optional) – If True, both the output and result will be computed in a differentiable way. Note that when strict is False, the result can not require gradients or be disconnected from the inputs. Defaults to False.

- strict ([bool](https://docs.python.org/3/builtins/functions.html#bool), optional) – If True, an error will be raised when we detect that there exists an input such that all the outputs are independent of it. If False, we return a Tensor of zeros as the hvp for said inputs, which is the expected mathematical value. Defaults to False.

Returns:

tuple with:

func_output (tuple of Tensors or Tensor): output of func(inputs)

hvp (tuple of Tensors or Tensor): result of the dot product with
the same shape as the inputs.

Return type:

output ([tuple](https://docs.python.org/3/builtins/stdtypes.html#tuple))

Example

```
>>> def pow_reducer(x):
...     return x.pow(3).sum()
>>> inputs = tensorplay.rand(2, 2)
>>> v = tensorplay.ones(2, 2)
>>> output = hvp(pow_reducer, inputs, v)
>>> output[0]
tensor(0.1448)
>>> output[1]
tensor([[2.0239, 1.6456],
        [2.4988, 1.4310]])
```

> **Note**
>
> This function is significantly slower than vhp due to backward mode AD constraints. If your function is twice continuously differentiable, then hvp = vhp.t(). So if you know that your function satisfies this condition, you should use vhp instead that is much faster with the current implementation.