Linear algebra: eigenvalues and systems
Eigenvalues satisfy det(A - λI) = 0. For a 2x2 matrix, λ₁ + λ₂ = trace and λ₁λ₂ = determinant. A system Ax = b has a unique solution when det(A) ≠ 0. Rank deficiency introduces free variables or inconsistency depending on the augmented matrix.
Check yourself: Did you verify the determinant condition before claiming a unique solution?
Calculus: multivariable and integral theorems
Gradient points in the direction of steepest ascent. Stokes' and divergence theorems convert between line/surface integrals and area/volume integrals. Apply Green's theorem for 2D: ∮(P dx + Q dy) = ∬(∂Q/∂x - ∂P/∂y) dA over a simply connected region.
Check yourself: Is the region simply connected and the field continuously differentiable?
Ordinary differential equations
A first-order linear ODE dy/dx + P(x)y = Q(x) has integrating factor e^(∫P dx). For second-order constant-coefficient equations, the characteristic equation determines complementary solutions: distinct real roots give exponentials, repeated roots add a polynomial factor, complex roots give sinusoids.
Check yourself: Have you applied the initial or boundary conditions after finding the general solution?
Probability and statistics
For independent events, P(A ∩ B) = P(A)P(B). Bayes' theorem inverts conditional probability: P(A|B) = P(B|A)P(A)/P(B). The mean of a continuous distribution is ∫x f(x) dx; variance is E(X²) - [E(X)]². The normal distribution is characterized by mean and variance alone.
Check yourself: Are the events independent, or did you assume independence without justification?
Numerical methods
Newton-Raphson iterates x_{n+1} = x_n - f(x_n)/f'(x_n), converging quadratically near a simple root when f'(x_n) ≠ 0. The trapezoidal rule approximates ∫f dx with error proportional to h². Simpson's 1/3 rule requires an even number of intervals and has error proportional to h⁴.
Check yourself: Is f'(x_n) near zero, which would cause divergence or oscillation?