# The Theory of Causal Fermion Systems

## Notions of Entropy

### Prerequisites

### Related Topics

## Notions of Entropy

Causal fermion systems give rise to various notions entropy.

In [entropy21] a notion of entropy was introduced for causal fermion systems. This entropy is a measure of the state of disorder of a causal fermion system at a given time compared to the vacuum. It should be regarded as a quantum entropy. It also gives rise to a corresponding entanglement entropy.

Moreover, in [FJLMM24] *fermionic entropies* are introduced for causal fermion systems and are illustrated in several examples. More precisely, the von Neumann entropy, the entanglement entropy and a relative entropy are defined. These definitions make use of the entropy formulas for quasi-free fermionic states in terms of the reduced one-particle density operator constructed from the kernel of the fermionic projector.

### Felix Finster

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