Notes, summaries, assignments, exams, and problems for Mathematics

Linear Algebra and Matrix Properties

1. State one property of a square matrix that will guarantee it is singular.
   There are many possibilities, some of which are: having determinant 0, having dependent columns, having dependent rows, and having a non-trivial null-space.
2. Can a system of linear equations have exactly 2 solutions?
   No, a system of linear equations can have either 0, 1, or infinitely many solutions.
3. What advantages does the Cholesky factorization have over Gaussian elimination?
   The Cholesky factorization only requires about half the operations of Gaussian elimination, and pivoting is never required for stability.
4. Can an n × n positive-definite matrix be singular? What about an n × n symmetric positive-definite matrix?
   The condition

Process of Contouring and Isolines

Isolines connect points of equal measurements. Contours indicate elevation and changes in elevation, such as slopes and hills. Isotherms connect points of equal temperature and tell us the current state of the atmosphere.

Rules for Drawing Isolines

• Connect points of equal value.
• Never cross one another.
• Never touch.
• The interval for isolines is the same for the whole map.

Contours show us slopes, valleys, and hills.

Topographic Profiles and Vertical Exaggeration

A profile is a topographic profile representing the lay of the land from the side. To draw a profile, a transect (straight line) is drawn between two points to show hills, valleys, and slopes.

Vertical Exaggeration is a mathematical representation used to ensure... Continue reading "Mapping Techniques and Geographic Data Analysis" »

This step-by-step analysis covers Problem Sets 6-9, emphasizing key concepts from Problem Sets 7 and 8, essential for your final exam.

Problem Set 6: Product Differentiation & Merger Impacts

1. Why Bertrand Does Not Equal Marginal Cost in Reality

• Firms may experience:

  • Capacity constraints

  • Brand loyalty (differentiated products)

  • Reputational concerns or switching costs

2. Bertrand Competition with Differentiated Products

• Demand:

  • Q_M = 1000 - 200P_M + 100P_B

  • Q_B = 1000 - 200P_B + 100P_M

• Steps:

  1. Plug in rival's price to derive inverse demand.

  2. Derive Marginal Revenue (MR); set MR = Marginal Cost (MC) = 4.

  3. Solve for the best response price.

  4. Set both best responses equal to solve for the Nash Equilibrium (NE).

  5. Calculate quantity, profit, and price-cost margin.

Chapter 2: Predetermined Overhead Rate

Predetermined Overhead Rate = Estimated Total Manufacturing Overhead (MOH) / Estimated Total MOH Driver (e.g., Direct Labor hours, Direct Labor costs, Machine Hours)

Prime Cost = Direct Materials + Direct Manufacturing Labor

Conversion Cost = Direct Manufacturing Labor + Indirect Manufacturing Overhead

Cost Accumulation: Data is collected in an organized way (also known as cost pools).

Cost Assignment: Systematically links an actual cost pool to a distinct cost object (e.g., Tires, engine, labor assigned to car cost).

Activity Base: Examples include kilometers driven in a car, units produced, units sold, machine hours.

Product Cost: Costs tied to creating a product (Direct Materials, Direct Labor, Manufacturing... Continue reading "Cost Accounting Essentials: Key Concepts and Calculations" »

Statistical Measures and Causal Inference Concepts

Measures of Dispersion and Relationship

Variance

Variance: Estimates how far a set of numbers (random) are spread out from their mean value.

Covariance

Covariance: The relationship between two variables.

• Cov = 0: Unsure of the relationship.
• Cov > 0: Suggests Y will be above average when X is above average.
• Cov < 0: Suggests Y will be below average when X is above average.

The formula for variance is often expressed as: $\mathbb{E}[X^2] - (\mathbb{E}[X])^2$ (where $\mathbb{E}$ is the Expected Value).

The formula for covariance between two variables $X$ and $Y$ is: $\mathbb{E}[(X - \mathbb{E}[X])(Y - \mathbb{E}[Y])]$

Pearson's Correlation Coefficient

Standardizes covariance between -1 and 1:

Pearson’s

Regression Statistics

• Multiple R: Coefficient of correlation (0.099). 9.9% of variability in Y is connected with 9.95% of variability in X.
• R-squared: Coefficient of determination (0.0099). 0.99% of variance in Y is explained by our regression model.
• Standard Error: The prediction of Y made using our model will differ from reality by approximately [number].
• Observations: The model contains [x] units.

Intercept (B0)

Coefficients: If we do not take X into consideration, Y will be [..].

T-stat: Calculated as (coefficient / standard error).

P-value: Level of risk is nearly 0, indicating a 99.99% probability.

Lower/Upper 95%: We are 95% confident that our coefficient B0 falls between 27.4 and 30.8.

Age (B1)

Coefficients: If X increases by 1 year, Y will increase... Continue reading "Regression Analysis Statistics and Interpretation Explained" »

Data Science, Machine Learning, and Artificial Intelligence

AdaBoost: Adaptive Boosting Algorithm Explained

AdaBoost (Adaptive Boosting) is a classic and widely used boosting algorithm that focuses on correcting the errors of preceding weak learners (typically decision trees). It works by iteratively adjusting the weights of the training data points.

How AdaBoost Works

1. Initial Weights: AdaBoost starts by assigning equal weights to all the training data points.
2. Train a Weak Learner: A "weak" learner (a model that performs slightly better than random chance, like a decision stump) is trained on the dataset using the current weights.
3. Calculate Error and Performance: The error rate of the weak learner is calculated based on the instances it misclassified. A measure of the weak learner's performance (often called

1. Understand the Problem

The first step to solving a two-step algebraic equation is to clearly write down the problem. This helps you visualize the solution process. For our example, we will work with the equation: -4x + 7 = 15.

2. Isolate the Variable Term Using Addition or Subtraction

The next step is to isolate the variable term (e.g., "-4x") on one side of the equation and the constants (whole numbers) on the other. To achieve this, you'll use the Additive Inverse. Find the opposite of the constant term on the same side as the variable. In our example, the constant is +7, so its additive inverse is -7.

Subtract 7 from both sides of the equation to cancel out the "+7" on the variable's side. Write "-7" below the 7 on the left side and below... Continue reading "Mastering Two-Step Algebraic Equations" »