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# tf.contrib.kernel_methods.RandomFourierFeatureMapper

Class that implements Random Fourier Feature Mapping (RFFM) in TensorFlow.

The RFFM mapping is used to approximate the Gaussian (RBF) kernel:

$$(exp(-||x-y||_2^2 / (2 * \sigma^2))$$

The implementation of RFFM is based on the following paper: "Random Features for Large-Scale Kernel Machines" by Ali Rahimi and Ben Recht. (link: https://people.eecs.berkeley.edu/~brecht/papers/07.rah.rec.nips.pdf)

The mapping uses a matrix $$\Omega \in R^{d x D}$$ and a bias vector $$b \in R^D$$ where $$d$$ is the input dimension (number of dense input features) and $$D$$ is the output dimension (i.e., dimension of the feature space the input is mapped to). Each entry of $$\Omega$$ is sampled i.i.d. from a (scaled) Gaussian distribution and each entry of $$b$$ is sampled independently and uniformly from [0, $$2 * \pi$$].

For a single input feature vector $$x \in R^d$$, its RFFM is defined as:

$$\sqrt(2/D) * cos(x * \Omega + b)$$

where $$cos$$ is the element-wise cosine function and $$x, b$$ are represented as row vectors. The aforementioned paper shows that the linear kernel of RFFM-mapped vectors approximates the Gaussian kernel of the initial vectors.

input_dim The dimension (number of features) of the tensors to be mapped.
output_dim The output dimension of the mapping.
stddev The standard deviation of the Gaussian kernel to be approximated. The error of the classifier trained using this approximation is very sensitive to this parameter.
seed An integer used to initialize the parameters ($$\Omega$$ and $$b$$) of the mapper. For repeatable sequences across different invocations of the mapper object (for instance, to ensure consistent mapping both at training and eval/inference if these happen in different invocations), set this to the same integer.
name name for the mapper object.

input_dim

name Returns a name for the RandomFourierFeatureMapper instance.

If the name provided in the constructor is None, then the object's unique id is returned.

output_dim Returns the output dimension of the mapping.

## Methods

### map

View source

Maps each row of input_tensor using random Fourier features.

Args
input_tensor a Tensor containing input features. It's shape is [batch_size, self._input_dim].

Returns
A Tensor of shape [batch_size, self._output_dim] containing RFFM-mapped features.

Raises
InvalidShapeError if the shape of the input_tensor is inconsistent with expected input dimension.

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