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Gaussians Pieter Abbeel UC Berkeley EECS

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Gaussians

Pieter Abbeel

UC Berkeley EECS

Many slides adapted from Thrun, Burgard and Fox, Probabilistic Robotics

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- Univariate Gaussian
- Multivariate Gaussian
- Law of Total Probability
- Conditioning (Bayes’ rule)
Disclaimer: lots of linear algebra in next few lectures. See course homepage for pointers for brushing up your linear algebra.

In fact, pretty much all computations with Gaussians will be reduced to linear algebra!

- Gaussian distribution with mean , and standard deviation s:

- Densities integrate to one:
- Mean:
- Variance:

- Classical CLT:
- Let X1, X2, … be an infinite sequence of independent random variables with E Xi = , E(Xi - )2 = 2
- Define Zn = ((X1 + … + Xn) – n ) / ( n1/2)
- Then for the limit of n going to infinity we have that Zn is distributed according to N(0,1)

- Crude statement: things that are the result of the addition of lots of small effects tend to become Gaussian.

(integral of vector = vector of integrals of each entry)

(integral of matrix = matrix of integrals of each entry)

- = [1; 0]
- = [1 0;0 1]

- = [-.5; 0]
- = [1 0; 0 1]

- = [-1; -1.5]
- = [1 0; 0 1]

- = [0; 0]
- = [1 0 ; 0 1]

- = [0; 0]
- = [.6 0 ; 0 .6]

- = [0; 0]
- = [2 0 ; 0 2]

Multi-variate Gaussians: examples

- = [0; 0]
- = [1 0; 0 1]

- = [0; 0]
- = [1 0.5; 0.5 1]

- = [0; 0]
- = [1 0.8; 0.8 1]

- = [0; 0]
- = [1 0; 0 1]

- = [0; 0]
- = [1 0.5; 0.5 1]

- = [0; 0]
- = [1 0.8; 0.8 1]

- = [0; 0]
- = [1 -0.5 ; -0.5 1]

- = [0; 0]
- = [1 -0.8 ; -0.8 1]

- = [0; 0]
- = [3 0.8 ; 0.8 1]

- Consider a multi-variate Gaussian and partition random vector into (X, Y).

- Precision matrix
- Straightforward to verify from (1) that:
- And swapping the roles of ¡ and §:

(1)

We integrate out over y to find the marginal:

Hence we have:

Note: if we had known beforehand that p(x) would be a Gaussian distribution, then we could have found the result more quickly. We would have just needed to find and , which we had available through

If

Then

We have

Hence we have:

- Mean moved according to correlation and variance on measurement
- Covariance §XX | Y = y0 does not depend on y0

If

Then