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Course 88790

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The Hilbert matrix with entries is notoriously ill-conditioned. For , . A scientific computation uses to model a system where input measurements have 0.01% relative error.

What is the expected relative error in the output?

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A climate model predicts temperature () using three variables: CO₂ concentration (), methane concentration (), and total greenhouse gas index (). Historically, (strong linear relationship). The prediction system where shows unstable predictions.

Why does the strong correlation between

and cause prediction instability?
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In electrical circuit analysis using Kirchhoff's laws, systems of equations arise where coefficients can differ by many orders of magnitude when components have vastly different resistances.

An electrical circuit contains resistors

Ω, Ω, and Ω connected in a network. The resulting system matrix has entries spanning 7 orders of magnitude. Why is partial pivoting essential for this system?
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An economist models relationships between unemployment rate (), inflation (), and GDP growth () using the linear system , where . Historical data shows these variables are strongly correlated (when increases, decreases proportionally, and adjusts similarly), making the equations nearly linearly dependent.

Predictions using this model yield wildly different results each month despite similar input data. Why?

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Gaussian elimination's forward elimination phase performs multiplications for an matrix. This sum simplifies to .

Calculate the exact number of multiplications required for forward elimination on a

matrix using the formula .
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Partial pivoting requires identifying the largest absolute value in the current column and swapping rows before performing elimination. For column 1, we compare , , ..., .

Perform the first step of Gaussian elimination with partial pivoting on:

After row interchange and one elimination step, what is the value of element

(row 2, column 1)?
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Gaussian elimination transforms a system into upper triangular form through row operations. The elimination process proceeds column by column, eliminating entries below the diagonal.

Consider the following Python code for Gaussian elimination:

def gauss_elim(A, b):

n = len(A)

for k in range(n):

for i in range(k, n):

factor = A[i][k] / A[k][k]

for j in range(k, n):

A[i][j] -= factor * A[k][j]

b[i] -= factor * b[k]

return A, b

Why does this code fail to produce the correct upper triangular matrix?

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Gaussian elimination involves dividing by pivot elements and subtracting rows. Several numerical issues can arise: subtractive cancellation when similar-magnitude values are subtracted, absorption when adding values of vastly different magnitudes, overflow/underflow when intermediate values exceed representable range, and error amplification when operations magnify small floating-point errors.

The following code performs Gaussian elimination without pivoting:

def gauss_no_pivot(A, b):

n = len(A)

for k in range(n-1):

for i in range(k+1, n):

m = A[i][k] / A[k][k]

for j in range(k, n):

A[i][j] -= m * A[k][j]

b[i] -= m * b[k]

return A, b

When applied to a matrix with

and other elements near 1, what is the primary consequence?
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In IEEE 754 double precision, the mantissa has 52 bits of precision, giving machine epsilon . During Gaussian elimination, operations include division (computing multipliers), multiplication (scaling rows), and subtraction (eliminating entries). Each operation can introduce or amplify floating-point errors depending on operand magnitudes.

Why is dividing by a large-magnitude number more numerically stable than dividing by a small-magnitude number during Gaussian elimination?

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The Gauss-Seidel method updates variables sequentially, immediately using newly computed values:

A student implements Gauss-Seidel for a 3

3 system as follows:

def gauss_seidel(A, b, x, n_iter):

n = len(b)

for k in range(n_iter):

for i in range(n-1, -1, -1):

sigma = sum(A[i][j]*x[j] for j in range(n) if j != i)

x[i] = (b[i] - sigma) / A[i][i]

return x

The code updates variables in reverse order (

, then , then ) instead of forward order. How does this affect the method?
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