Recurrent neural networks are neural architectures designed for sequential data processing, applied in particle energy measurement, molecular dynamics, and memory consolidation studies.The conductor's baton hangs in the air—the musician waits, remembering the previous motif. A recurrent network iterates over sequence tokens (characters or words) and after each updates the hidden state (a vector of numbers). This state is a compressed description of everything seen so far. Thanks to the feedback loop, the network can capture dependencies between distant elements: for example, case agreement in a long German sentence.
The foundations trace back to the work of John Hopfield (1982) and networks with feedback. In 1986, David Rumelhart et al. applied backpropagation to RNNs. Practical application was limited by the vanishing/exploding gradient problem. In 1997, Sepp Hochreiter and Jürgen Schmidhuber proposed LSTM (long short-term memory) with gated units—valves—solving long-term dependency issues. In 2014, Kyunghyun Cho et al. introduced GRU. Since the mid-2010s, RNNs have been gradually replaced by transformers but remain important in streaming tasks.
At step t, the RNN receives the input vector x_t and the previous hidden state h_{t-1}, computes the new state: h_t = \tanh(W_x x_t + W_h h_{t-1} + b). The vector h_t is fed to the output layer to predict y_t and passed to the next step. The parameters W_x, W_h, b are shared across all steps, enabling generalization to sequences of different lengths. Training is done by unfolding the network in time (backpropagation through time). LSTM adds a memory cell c_t and three gates: forget, input, and output, which regulate the flow of information.
Text autocomplete on a phone, machine translation, smart speaker processing of long phrases.
LSTM networks, created in 1997, remained the primary tool for machine translation for almost 20 years, until transformers supplanted them—yet they are still used in millions of smartphones for text autocomplete.













