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Covariance between a standard normal and its square

Question

Let ZZ be standard normal, and define X=ZX=Z and Y=Z2Y=Z^2. What is Cov⁡(X,Y)\operatorname{Cov}(X,Y)?

Short answer

00. Cov⁡(Z,Z2)=E[Z3]−E[Z] E[Z2]\operatorname{Cov}(Z,Z^2)=\mathbb{E}[Z^3]-\mathbb{E}[Z]\,\mathbb{E}[Z^2], and both E[Z3]\mathbb{E}[Z^3] and E[Z]\mathbb{E}[Z] vanish by symmetry of the normal. This is a classic example of uncorrelated yet dependent variables.

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