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Answers for “条件熵 H(X|Y) 与联合熵 H(X,Y) 和边缘熵 H(Y) 有什么关系?”

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Conditional entropy is the difference between the joint entropy of X and Y and the marginal entropy of Y. This relationship is expressed by the entropy chain rule: H(X|Y) = H(X,Y)−H(Y)H(X,Y) - H(Y).

Conditions: X and Y are discrete random variables with well-defined joint and marginal entropies.

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Conditional entropy H(X|Y) measures the average remaining uncertainty or surprise about a random variable X after another random variable Y has been observed. It is defined as the expected value of the conditional self-information h(x|y) over the joint distribution of X and Y.

Conditions: X and Y are discrete random variables.; The sum is taken over the joint alphabet Ax×AyA_x \times A_y.

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Joint entropy H(X,Y)H(X,Y) is not equal to H(X)+H(Y)H(X) + H(Y) because adding the marginal entropies double-counts the mutual information I(X;Y) shared between the variables. The correct relationship is H(X,Y)=H(X)+H(Y)−IH(X,Y) = H(X) + H(Y) - I(X;Y).

Conditions: X and Y are random variables that may share information (i.e., they are not necessarily independent).

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The exercise asks students to implement a Matlab function conditionalentropy(p) that computes the conditional entropy of X given Y from a 2D joint probability matrix p(x,y)p(x,y). It includes specific test cases to verify correctness, such as deterministic and uniform distributions.

Conditions: Input p is assumed to be a 2D joint probability array p(x,y)p(x,y).; Students may reuse existing entropy() and jointentropy() routines.