Physlib

PhyslibAlpha.ProbabilisticTheory.Effect.Convex

Convexity and mixtures of effects

i. Overview

The effect interval `[0, 1]` is convex: randomizing between two effects with some probability gives back an effect. Physically, flipping a biased coin to decide which of two measurements to actually run is itself a legitimate measurement.

ii. Key results

  • `Effect.convex` : the effect interval is convex.
  • `Effect.mix` : randomize between two effects with a given probability.

iii. Table of contents

  • A. Convexity and mixtures of effects

A. Convexity and mixtures of effects

3 declarations

theorem

Convexity of the set of effects

The set of effects in a type EE, denoted as `Effect E` and defined as the order interval [0,1]={A∈E∣0≤A≤1}[0, 1] = \{A \in E \mid 0 \le A \le 1\}, is a convex set over the real numbers R\mathbb{R}.

definition

Mixture of effects te+(1−t)ft e + (1 - t) f

For a type EE with a partial order, a zero element 00, and a unit element 11, let ee and ff be **effects** (elements in the order interval [0,1]⊆E[0, 1] \subseteq E). Given a value tt in the unit interval [0,1][0, 1], this function returns the **mixture** of ee and ff, which is the effect defined by the convex combination te+(1−t)ft e + (1 - t) f. This represents randomizing between the two effects with probability tt of testing the first.

theorem

Mixture of effects is the convex combination te+(1−t)ft e + (1 - t) f

For any two effects e,fe, f in the interval [0,1]⊆E[0, 1] \subseteq E and a probability tt in the unit interval [0,1]⊆R[0, 1] \subseteq \mathbb{R}, the mixture of ee and ff with parameter tt is equal to the convex combination: mix(e,f,t)=t⋅e+(1−t)⋅f \text{mix}(e, f, t) = t \cdot e + (1 - t) \cdot f where the equality holds for the corresponding elements in the underlying type EE.