 Activation Functions

# Gaussian Error Linear Units

Introduced by Hendrycks et al. in Gaussian Error Linear Units (GELUs)

The Gaussian Error Linear Unit, or GELU, is an activation function. The GELU activation function is $x\Phi(x)$, where $\Phi(x)$ the standard Gaussian cumulative distribution function. The GELU nonlinearity weights inputs by their percentile, rather than gates inputs by their sign as in ReLUs ($x\mathbf{1}_{x>0}$). Consequently the GELU can be thought of as a smoother ReLU.

$$\text{GELU}\left(x\right) = x{P}\left(X\leq{x}\right) = x\Phi\left(x\right) = x \cdot \frac{1}{2}\left[1 + \text{erf}(x/\sqrt{2})\right],$$ if $X\sim \mathcal{N}(0,1)$.

One can approximate the GELU with $0.5x\left(1+\tanh\left[\sqrt{2/\pi}\left(x + 0.044715x^{3}\right)\right]\right)$ or $x\sigma\left(1.702x\right),$ but PyTorch's exact implementation is sufficiently fast such that these approximations may be unnecessary. (See also the SiLU $x\sigma(x)$ which was also coined in the paper that introduced the GELU.)

GELUs are used in GPT-3, BERT, and most other Transformers.

#### Papers

Paper Code Results Date Stars

Language Modelling 88 13.00%
Text Generation 27 3.99%
Text Classification 23 3.40%
Sentiment Analysis 17 2.51%
Speech Recognition 14 2.07%
Machine Translation 14 2.07%
Knowledge Distillation 13 1.92%
Few-Shot Learning 12 1.77%

#### Components

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