$$y = \sigma \left( \sum_{i=1}^n W_i x_i + b \right)$$
Nonlinear affine composition. Bedrock of the Universal Approximation Theorem (Cybenko, 1989; Hornik, 1991): any continuous function over compact subsets of $\mathbb{R}^n$ can be arbitrarily approximated by a single hidden layer.
$$(f * g)(t) = \sum_{\tau} f(\tau) g(t - \tau)$$
Leverages spatial locality and translation equivariance via shared receptive field kernels (LeCun, 1989). Drastically reduces free parameters in high-dimensional tensors (2D/3D images).
$$C_t = f_t \odot C_{t-1} + i_t \odot \tilde{C}_t$$
Resolves the vanishing gradient dilemma through multiplicative gating mechanisms ($f_t, i_t, o_t$) and an additive linear memory highway $C_t$ (Hochreiter & Schmidhuber, 1997).
$$\mathcal{L}_{\text{ELBO}} = \mathbb{E}[\log p(x|z)] - D_{\text{KL}}(q(z|x) \parallel p(z))$$
Learns a smooth continuous latent manifold by mapping inputs to Gaussian distributions $\mathcal{N}(\mu, \Sigma)$. Employs the reparameterization trick ($z = \mu + \sigma \odot \epsilon$) for stochastic backprop.
$$\min_G \max_D \mathbb{E}_x[\log D(x)] + \mathbb{E}_z[\log(1 - D(G(z)))]$$
Zero-sum minimax game between two competing networks: a Generator synthesizing candidates from latent noise and a Discriminator estimating real-vs-fake probability (Goodfellow et al., 2014).
$$\text{Attention}(Q, K, V) = \text{softmax}\left(\frac{QK^T}{\sqrt{d_k}}\right)V$$
Replaces sequential recurrence with parallel pairwise scaled dot-product attention across all tokens simultaneously (Vaswani et al., 2017). The foundation of LLMs and Vision Transformers (ViT).
$$\mathcal{L}_{\text{pinn}} = \mathcal{L}_{\text{data}} + \lambda_{\text{edp}} \|\mathcal{N}[u_\theta]\|_2^2 + \lambda_{\text{bc}} \mathcal{L}_{\text{bc}}$$
Embeds partial differential operators (PDEs) directly into the loss objective via continuous automatic differentiation (Raissi et al., 2019). Operates completely mesh-free.
$$q(x_t | x_{t-1}) = \mathcal{N}\left(x_t; \sqrt{1 - \beta_t} x_{t-1}, \beta_t \mathbf{I}\right)$$
Inspired by non-equilibrium statistical thermodynamics (Sohl-Dickstein, 2015; Ho et al., 2020). Gradually add Gaussian noise to signals and train a neural network to invert the diffusion trajectory step-by-step.