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Lecture 2: Math Primer. Machine Learning CUNY Graduate Center. Today. Probability and Statistics Naïve Bayes Classification Linear Algebra Matrix Multiplication Matrix Inversion Calculus Vector Calculus Optimization Lagrange Multipliers. Classical Artificial Intelligence.

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Lecture 2 math primer
Lecture 2: Math Primer

Machine Learning

CUNY Graduate Center


  • Probability and Statistics

    • Naïve Bayes Classification

  • Linear Algebra

    • Matrix Multiplication

    • Matrix Inversion

  • Calculus

    • Vector Calculus

    • Optimization

    • Lagrange Multipliers

Classical artificial intelligence
Classical Artificial Intelligence

Expert Systems

Theorem Provers



Largely characterized by determinism.

Modern artificial intelligence
Modern Artificial Intelligence

Fingerprint ID

Internet Search

Vision – facial ID, object recognition

Speech Recognition



Statistical modeling to generalize from data.

Two caveats about statistical modeling
Two Caveats about Statistical Modeling

Black Swans

The Long Tail

Black swans
Black Swans

  • In the 18th Century, black swans were discovered in Western Australia

  • Black Swans are rare, sometimes unpredictable events, that have extreme impact

  • Almost all statistical models underestimate the likelihood of unseen events.

In the 17th Century, all known swans were white.

Based on evidence, it is impossible for a swan to be anything other than white.

The long tail
The Long Tail

  • Many events follow an exponential distribution

  • These distributions have a very long “tail”.

    • I.e. A large region with significant probability mass, but low likelihood at any particular point.

  • Often, interesting events occur in the Long Tail, but it is difficult to accurately model behavior in this region.

Boxes and balls
Boxes and Balls

2 Boxes, one red and one blue.

Each contain colored balls.

Boxes and balls1
Boxes and Balls

Suppose we randomly select a box, then randomly draw a ball from that box.

The identity of the Box is a random variable, B.

The identity of the ball is a random variable, L.

B can take 2 values, r, or b

L can take 2 values, g or o.

Boxes and balls2
Boxes and Balls

Given some information about B and L, we want to ask questions about the likelihood of different events.

What is the probability of selecting an apple?

If I chose an orange ball, what is the probability that I chose from the blue box?

Some basics
Some basics

  • The probability (or likelihood) of an event is the fraction of times that the event occurs out of n trials, as n approaches infinity.

  • Probabilities lie in the range [0,1]

  • Mutually exclusive events are events that cannot simultaneously occur.

    • The sum of the likelihoods of all mutually exclusive events must equal 1.

  • If two events are independent then,

    p(X, Y) = p(X)p(Y)

    p(X|Y) = p(X)

Joint probability p x y
Joint Probability – P(X,Y)

A Joint Probability function defines the likelihood of two (or more) events occurring.

Let nij be the number of times event i and event j simultaneously occur.


Consider the probability of X irrespective of Y.

The number of instances in column j is the sum of instances in each cell

Therefore, we can marginalize or “sum over” Y:

Conditional probability
Conditional Probability

  • Consider only instances where X = xj.

  • The fraction of these instances where Y = yi is the conditional probability

    • “The probability of y given x”

Sum and product rules
Sum and Product Rules

Sum Rule

Product Rule

In general, we’ll refer to a distribution over a random variable as p(X) and a distribution evaluated at a particular value as p(x).

Interpretation of bayes rule
Interpretation of Bayes Rule




Prior: Information we have before observation.

Posterior: The distribution of Y after observing X

Likelihood: The likelihood of observing X given Y

Boxes and balls with bayes rule
Boxes and Balls with Bayes Rule

  • Assuming I’m inherently more likely to select the red box (66.6%) than the blue box (33.3%).

  • If I selected an orange ball, what is the likelihood that I selected the red box?

    • The blue box?

Na ve bayes classification
Naïve Bayes Classification

This is a simple case of a simple classification approach.

Here the Box is the class, and the colored ball is a feature, or the observation.

We can extend this Bayesian classification approach to incorporate more independent features.

Na ve bayes classification1
Naïve Bayes Classification

Some theory first.

Na ve bayes classification2
Naïve Bayes Classification

Assuming independent features simplifies the math.

Continuous probabilities
Continuous Probabilities

So far, X has been discrete where it can take one of M values.

What if X is continuous?

Now p(x) is a continuous probability density function.

The probability that x will lie in an interval (a,b) is:

Expected values
Expected Values

Given a random variable, with a distribution p(X), what is the expected value of X?

Multinomial distribution
Multinomial Distribution

If a variable, x, can take 1-of-K states, we represent the distribution of this variable as a multinomial distribution.

The probability of x being in state k is μk

Expected value of a multinomial
Expected Value of a Multinomial

The expected value is the mean values.

Gaussian distribution
Gaussian Distribution

One Dimension


How machine learning uses statistical modeling
How machine learning uses statistical modeling

  • Expectation

    • The expected value of a function is the hypothesis

  • Variance

    • The variance is the confidence in that hypothesis


The variance of a random variable describes how much variability around the expected value there is.

Calculated as the expected squared error.


The covariance of two random variables expresses how they vary together.

If two variables are independent, their covariance equals zero.

Linear algebra
Linear Algebra

  • Vectors

    • A one dimensional array.

    • If not specified, assume x is a column vector.

  • Matrices

    • Higher dimensional array.

    • Typically denoted with capital letters.

    • n rows by m columns


Transposing a matrix swaps columns and rows.


Transposing a matrix swaps columns and rows.


  • Matrices can be added to themselves iff they have the same dimensions.

    • A and B are both n-by-m matrices.


  • To multiply two matrices, the inner dimensions must be the same.

    • An n-by-m matrix can be multiplied by an m-by-k matrix


  • The inverse of an n-by-n or squarematrix A is denoted A-1, and has the following property.

  • Where I is the identity matrix is an n-by-n matrix with ones along the diagonal.

    • Iij = 1 iffi = j, 0 otherwise

Identity matrix
Identity Matrix

Matrices are invariant under multiplication by the identity matrix.

Lecture 2 math primer

The norm of a vector,x, represents the euclidean length of a vector.

Positive definite ness
Positive Definite-ness

  • Quadratic form

    • Scalar

    • Vector

  • Positive Definite matrix M

  • Positive Semi-definite


Derivatives and Integrals



A derivative of a function defines the slope at a point x.


Integration is the inverse operation of derivation (plus a constant)

Graphically, an integral can be considered the area under the curve defined by f(x)

Vector calculus
Vector Calculus

Derivation with respect to a matrix or vector


Change of Variables with a Vector

Derivative w r t a vector
Derivative w.r.t. a vector

Given a vector x, and a function f(x), how can we find f’(x)?

Derivative w r t a vector1
Derivative w.r.t. a vector

Given a vector x, and a function f(x), how can we find f’(x)?

Example derivation1
Example Derivation

Also referred to as the gradient of a function.

Useful vector calculus identities
Useful Vector Calculus identities

Scalar Multiplication

Product Rule

Useful vector calculus identities1
Useful Vector Calculus identities

Derivative of an inverse

Change of Variable


Have an objective function that we’d like to maximize or minimize, f(x)

Set the first derivative to zero.

Optimization with constraints
Optimization with constraints

  • What if I want to constrain the parameters of the model.

    • The mean is less than 10

  • Find the best likelihood, subject to a constraint.

  • Two functions:

    • An objective function to maximize

    • An inequality that must be satisfied

Lagrange multipliers
Lagrange Multipliers

Find maxima of f(x,y) subject to a constraint.

General form
General form


Subject to:

Introduce a new variable, and find a maxima.



Subject to:

Introduce a new variable, and find a maxima.


Now have 3 equations with 3 unknowns.


Eliminate Lambda

Substitute and Solve

Why does machine learning need these tools
Why does Machine Learning need these tools?

  • Calculus

    • We need to identify the maximum likelihood, or minimum risk. Optimization

    • Integration allows the marginalization of continuous probability density functions

  • Linear Algebra

    • Many features leads to high dimensional spaces

    • Vectors and matrices allow us to compactly describe and manipulate high dimension al feature spaces.

Why does machine learning need these tools1
Why does Machine Learning need these tools?

  • Vector Calculus

    • All of the optimization needs to be performed in high dimensional spaces

    • Optimization of multiple variables simultaneously – Gradient Descent

    • Want to take a marginal over high dimensional distributions like Gaussians.

Next time
Next Time

Linear Regression and Regularization

Read Chapter 1.1, 3.1, 3.3