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Characteristic function of a multivariate normal law

Statement

Assume AC. If X∼Nd(m,Σ), then ΦX(t):=Eeit⋅X=exp⁡(it⋅m−tTΣt/2)(t∈Rd). This transform uniquely determines the law, including singular Sigma.

Facts & Assumptions

[F1]

Every linear projection has the specified scalar normal law, and the vector law exists. Multivariate normal law, including singular covariance.

[F2]

A scalar normal has transform exp⁡(ims−σ2s2/2). Characteristic function of a normal law.

[F3]

Under AC scalar laws with equal characteristic functions agree. Uniqueness of a law from its characteristic function.

[F4]

Under AC all linear projection laws determine the Borel vector law. Cramer wold device.

Proof

Given: Assume AC. If X∼Nd(m,Σ), then ΦX(t):=Eeit⋅X=exp⁡(it⋅m−tTΣt/2)(t∈Rd). This transform uniquely determines the law, including singular Sigma.

1.1F1F2

For fixed t, [F1] makes t⋅X scalar normal of mean t⋅m and variance tTΣt. Evaluate its characteristic function from [F2] at scalar frequency one. This gives the displayed formula. If t=0 both sides are one; if tTΣt=0 the scalar law is the point mass at t⋅m and the same formula applies.

2.1step 1.1F1F3F4∎

Let Y have another Borel probability law with the same displayed vector transform. For every u and scalar s, φu⋅Y(s)=ΦY(su)=ΦX(su)=φu⋅X(s). Scalar uniqueness [F3] identifies each pair of projection laws. Then [F4] identifies the vector laws. Thus the construction in [F1] is independent of any realization choices. No determinant or inverse of Sigma is used. AC is inherited from [F1]–[F4]; in dimension zero the single possible law makes the assertion immediate.

Depends on

Used by

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