Multivariate Random Variables

mathStatica extends naturally to a multivariate setting. To illustrate, let us suppose that [Graphics:Images/index_gr_1.gif] and [Graphics:Images/index_gr_2.gif] have joint pdf [Graphics:Images/index_gr_3.gif] with support [Graphics:Images/index_gr_4.gif], [Graphics:Images/index_gr_5.gif]:

[Graphics:Images/index_gr_6.gif]

where parameter α is such that [Graphics:Images/index_gr_7.gif]. This is known as a Gumbel bivariate Exponential distribution. Here is a plot of [Graphics:Images/index_gr_8.gif]:

[Graphics:Images/index_gr_9.gif]

Fig. 1: A Gumbel bivariate Exponential pdf when [Graphics:Images/index_gr_10.gif]

Here is the cdf, namely [Graphics:Images/index_gr_11.gif]:

[Graphics:Images/index_gr_12.gif]
[Graphics:Images/index_gr_13.gif]

Here is [Graphics:Images/index_gr_14.gif], the covariance between [Graphics:Images/index_gr_15.gif] and [Graphics:Images/index_gr_16.gif]:

[Graphics:Images/index_gr_17.gif]
[Graphics:Images/index_gr_18.gif]

More generally, here is the variance-covariance matrix:

[Graphics:Images/index_gr_19.gif]
[Graphics:Images/index_gr_20.gif]

Here is the marginal pdf of [Graphics:Images/index_gr_21.gif]:

[Graphics:Images/index_gr_22.gif]
[Graphics:Images/index_gr_23.gif]

Here is the conditional pdf of [Graphics:Images/index_gr_24.gif], given [Graphics:Images/index_gr_25.gif]:

[Graphics:Images/index_gr_26.gif]
[Graphics:Images/index_gr_27.gif]
[Graphics:Images/index_gr_28.gif]

Here is the bivariate mgf [Graphics:Images/index_gr_29.gif]:

[Graphics:Images/index_gr_30.gif]
[Graphics:Images/index_gr_31.gif]
[Graphics:Images/index_gr_32.gif]

Differentiating the mgf is one way to derive moments. Here is the product moment [Graphics:Images/index_gr_33.gif]:

[Graphics:Images/index_gr_34.gif]
[Graphics:Images/index_gr_35.gif]

which we could otherwise have found directly with:

[Graphics:Images/index_gr_36.gif]
[Graphics:Images/index_gr_37.gif]

Multivariate transformations pose no problem to mathStatica either. For instance, let [Graphics:Images/index_gr_38.gif] and [Graphics:Images/index_gr_39.gif] denote transformations of [Graphics:Images/index_gr_40.gif] and [Graphics:Images/index_gr_41.gif]. Then our transformation equation is:

[Graphics:Images/index_gr_42.gif]

Using Transform, we can find the joint pdf of random variables [Graphics:Images/index_gr_43.gif] and [Graphics:Images/index_gr_44.gif], denoted [Graphics:Images/index_gr_45.gif]:

[Graphics:Images/index_gr_46.gif]
[Graphics:Images/index_gr_47.gif]

while the extremum of the domain of support of the new random variables are:

[Graphics:Images/index_gr_48.gif]
[Graphics:Images/index_gr_49.gif]