# Lecture 9 - ECE 771 Lecture 9 Dierential entropy Objective...

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ECE 771 Lecture 9 – Differential entropy Objective: Differential entropy is entropy defined for distributions with a con- tinuous random variable. We will learn about differential entropy. Reading: 1. Read chapter 9. Differential entropy A continuous random variable (for the purpose of this class) is one in which the distribution function F ( x ) is continuous: there are no jumps (discrete outputs). Definition 1 The differential entropy h ( X ) of a continuous random variable X with density f ( x ) and support S is h ( X ) = - Z S f ( x ) log f ( x ) dx. 2 Example 1 Let X ∼ U (0 , a ). (Uniform). Then h ( X ) = - Z a 0 1 a log 1 a dx = log a. Note that the differential entropy can be negative (if a < 1). This is why we refer to this as differential entropy, since entropy should always be positive. 2 Example 2 Normal: X 1 σ 2 π e - x 2 / 2 σ 2 = φ ( x ) . (We have zero mean, but that will make no difference on the entropy.) We will compute the entropy in nats. h ( X ) = - Z φ ln φdx = - Z φ ( x )[ - x 2 2 - ln 2 πσ 2 ] dx = EX 2 2 σ 2 + 1 2 ln 2 πσ 2 = 1 2 ln πeσ 2 nats 2 Having defined the differential entropy, we can now go through and define all the sorts of things we did before.

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