The Top 15 Determinants Interview Questions You Should Prepare For
Determinants are a fundamental topic in linear algebra that assess your ability to calculate the determinant of a matrix and understand its properties. Expect at least a few questions on determinants in any data science or machine learning interview, especially for roles requiring strong mathematical skills. I will walk through the 15 most common determinant interview questions with detailed explanations and examples to help you prepare.
- What is a determinant of a matrix?
The determinant of a matrix is a scalar value that gives important information about the matrix such as whether it is invertible or not. Geometrically, the determinant represents the volume scaling factor of the linear transformation represented by the matrix. To calculate the determinant of an n x n matrix, you take the sum of the products of matrix elements along each diagonal from top left to bottom right minus the products along each diagonal from bottom left to top right.
For example, for a 2 x 2 matrix:
det(A) = a11a22 – a12a21
And for a 3 x 3 matrix:
det(A) = a11*(a22a33 – a23a32) – a12*(a21a33 – a23a31) + a13*(a21a32 – a22a31)
- How is the determinant of a matrix calculated?
There are several ways to calculate the determinant of a matrix:
-
Use the diagonal product definition described above. This is easiest for small matrices.
-
Use cofactor expansion – calculate the determinant by expanding along a row or column using cofactors
-
Use Laplace expansion along the first row or column. The cofactors are not explicitly needed.
-
Use elementary row or column operations – perform operations like swapping or multiplying rows/columns to simplify the matrix before calculating the determinant
-
Use technology like computer programs and calculators to automatically calculate determinants.
- What are the properties of determinants?
Some key properties of determinants include:
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Determinant of a square matrix is a scalar (single number).
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Determinant of a triangular matrix is the product of the diagonal entries.
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Swapping two rows or columns changes the sign of the determinant.
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Multiplying a row or column by a scalar k multiplies the determinant by k.
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Determinant of the transpose of a matrix is the same as the original matrix.
-
Determinant of upper triangular matrices is the product of the diagonal entries.
- How do you calculate the determinant of a 3×3 matrix?
For a 3×3 matrix A:
det(A) = a11*(a22a33 – a23a32) – a12*(a21a33 – a23a31) + a13*(a21a32 – a22a31)
Where aij refers to the entry in the ith row and jth column. This uses the diagonal product definition by taking the sum of products across each diagonal.
For example, if:
A = [1 2 3]
[0 4 5]
[1 0 8]
Then:
det(A) = 1*(48 – 05) – 2*(08 – 15) + 3*(04 – 10) = 1*32 – 10 + 0 = 22
- What is a minor and cofactor of a matrix?
A minor of a matrix A is the determinant of the submatrix formed by deleting the ith row and jth column from A.
The cofactor of an element aij is defined as (-1)i+j times the minor of aij. It represents the signed scalar value obtained when the element aij is removed.
The cofactors can be used to calculate the determinant through cofactor expansion along a row or column.
For example, if:
A = [1 2 3]
[0 4 5]
[1 0 8]
The minor of a12 is:
|4 5| = 20
The cofactor of a12 is:
(-1)1+2 * 20 = -20
- How do you find the adjoint of a matrix?
The adjoint of a square matrix A, written as adj(A), is the transpose of the matrix of cofactors of A.
To get adj(A):
-
Calculate the matrix of cofactors of A by replacing each element by its cofactor.
-
Take the transpose of the cofactor matrix to obtain the adjoint matrix adj(A).
For example, the adjoint of:
A = [1 2]
[3 4]
Is:
adj(A) = [-4 2]
[ 3 -1]
- What is the relationship between the determinant and inverse of a matrix?
For a square matrix A:
- If det(A) ≠ 0, A is invertible and has a unique inverse A-1
- If det(A) = 0, A is not invertible and has no inverse
The inverse A-1 is given by:
A-1 = (1/det(A)) * adj(A)
Where adj(A) is the adjoint of A.
So the determinant being non-zero is a necessary and sufficient condition for a matrix to have an inverse.
- How do you check if a matrix is invertible?
To check if a square matrix A is invertible:
-
Calculate the determinant of A
-
If det(A) ≠ 0, A is invertible
-
If det(A) = 0, A is not invertible
So we just need to compute the determinant and check if it is non-zero. If yes, the matrix is guaranteed to have an inverse.
- What is a singular matrix?
A singular matrix is a square matrix with determinant equal to 0. This means the matrix is not invertible and has no inverse.
Geometrically, a singular matrix represents a degenerate linear transformation that scales the volume to 0.
Some examples of singular matrices:
- Null matrix (all entries 0)
- Non-square rectangular matrix
- Matrix with linearly dependent rows or columns
- Matrix with eigenvalue of 0
Identifying and handling singular matrices is important in many linear algebra operations.
- How do you diagonally dominate a matrix?
A matrix A is diagonally dominant if:
|aii| > Σj≠i |aij| for all i
Meaning the absolute value of each diagonal entry is greater than the sum of the absolute values of the other entries in its row.
To diagonally dominate a matrix:
- Interchange rows or columns to make the larger entries appear on the diagonal
- Multiply rows or columns by -1 as needed to make the diagonal entries positive
- Use elementary row operations to make the diagonal entries larger
Diagonal dominance helps improve the convergence of systems of linear equations.
- What is a triangular matrix?
A triangular matrix is a special matrix that has all its entries above or below the main diagonal as 0.
There are two types:
- Upper triangular matrix – has 0s below the diagonal
- Lower triangular matrix – has 0s above the diagonal
The determinants of upper and lower triangular matrices are simply the product of the diagonal entries. Triangular matrices have several applications in linear algebra and optimization.
- When is the determinant of a triangular matrix zero?
For both upper and lower triangular matrices, the determinant is zero if and only if any of the diagonal entries is zero.
This follows from the fact that the determinant of a triangular matrix is the product of only the diagonal entries.
So the determinant will be zero if and only if any of the diagonal entries is zero.
- How do you calculate the determinant recursively?
The determinant of an n x n matrix A can be calculated recursively using the relation:
det(A) = a1jdet(A1j) + (-1)1+ja2jdet(A2j) + … + (-1)n+janj*det(Anj)
Where:
- aij is the entry in the ith row and jth column
- Aij is the submatrix formed by removing the ith row and jth column from A
This uses Laplace expansion along the jth column and leverages the fact determinants satisfy a recursive relation with submatrices.
- What is a Vandermonde matrix?
A Vandermonde matrix is a matrix with the special form:
V = [1 x1 x12 … x1n-1]
[1 x2 x22 … x2n-1]
…
[1 xn xn2 … xnn-1]
Where the columns are powers of an arithmetic progression x1, x2, …, xn.
Vandermonde matrices have applications in polynomial interpolation and approximating functions. Their determinants also have a special closed form value.
- How do you check if a matrix is positive definite?
A symmetric n x n real matrix A is positive definite if:
- All leading principal minors are
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Table of Content
- The characteristic equation of a matrix is: a) det(A – λI) = 1 b) det(A – λI) = 0 c) A = λI d) A = I» The answer is b) det(A – λI) = 0. “An eigen vector is a vector that is represented by a matrix X.” When you multiply matrix A by vector X, the direction of the output matrix stays the same as the direction of vector X. A is any matrix, λ are Eigen values, and X is an Eigen vector. The formula is AX = λX. The equation used to figure out a matrix’s Eigenvalues is known as the matrix’s characteristic equation. Also known as characteristic polynomial. You can write |A – λI| = 0 for any square matrix A and any scalar λ. This equation shows that the matrix (A – λI)X = 0 turns X into the 0 vector. This also means that (A – λI) won’t have an opposite, so its determinant has to be 0. Its property is that det(A – λI) = 0. Its trace is Tr(A), which is equal to the sum of its diagonal elements.
- Which of the following is true about the Cayley-Hamilton theorem in linear algebra? a) Orthogonal Matrix b) Inverse Matrix c) Identity Matrix d) Square Matrix It is true that every square matrix with real or complex numbers meets its characteristic (eigen values) equation. If you turn a square matrix into a polynomial equation (det(A – λI)), you can get the answer 0. As an example, let’s say that A = 1 2 -1 2 and λI = [1 2 [1 0 -1 2] – λ* 0 1] |A – λI| will be a polynomial equation with degree n (since A is a n*n matrix). The polynomial equation λ^2 – 3λ 4 (degree 2, which is the same as the dimension of Matrix A 2*2) is also equal to zero. λ^2 – 3λ + 4 = 0″ .
- Diagonalization of matrix: a) An n*n matrix has n eigenvectors that are linearly independent of each other b) A is similar to a diagonal matrix. c) A = PDP^−1 for a matrix P that can be turned upside down and a diagonal matrix D d) All the Above. Answer – d) All the Above. “the act of taking a square matrix and turning it into a certain kind of diagonal matrix that has the same basic properties as the original matrix” You can also think of diagonalizing as a way to find the eigenvalues of a matrix, which are the diagonal elements of the matrix. ” .
- Which of these is not a square matrix? a) 3×3 matrix b) 2×3 matrix c) 4×3 matrix d) 1×2 matrix i. e. The size of a square matrix is n x n. Since the 1×2 matrix doesn’t have the same number of rows and columns as option D, it is not a square matrix. ” .
- Which of these is not a diagonal matrix? a) [[1,0,0], [0,2,0], [0,0,3]] b) [[1,2,3], [4,5,6], [7,8,9]] c) [[1,0], [0,1], [1,0]] d) [[0,0], [0,0], [0,0]] “A diagonal matrix is a square matrix where all the elements outside the main diagonal are zero.” Therefore, option a) is a diagonal matrix. ” .
- What size is AB if A is a 3×3 matrix and B is a 3×2 matrix? a) 3×2 b) 2×3 c) 3×3 d) 2×2 The answer is a) 3×2. “To multiply two matrices, the number of columns in the first matrix must be equal to the number of rows in the second matrix.” A has three rows and three columns, and B has three rows and two columns. This means that AB will have three rows and two columns. ” .
- The determinant of a matrix is a scalar value that can be positive, negative, or zero. It is not a) a scalar value; b) a negative value; c) the sum of its diagonal elements; or d) the product of its eigenvalues. The correct answer is c) the sum of its diagonal elements. If you want to know more about determinants, click here. It’s not the same as the sum of its diagonal parts. On the other hand, the trace of a matrix is the sum of its diagonal points. ” .
- If matrix A’s determinant is zero, what can we say about its inverse? Could it be that it exists and is unique? Could it be that it exists but may not be unique? Could it be that it does not exist? The correct answer is c) It does not exist. This is because you can only find the inverse of a matrix if the determinant is not zero. ” .
- Which of these is not a type of matrix? a) Identity matrix b) Diagonal matrix c) Scalar matrix d) Octagonal matrix The other options are types of matrices. ” .
- a) [[1,2], [3,4]] b) [[2,3,4], [5,6,7], [8,9,10]] c) [[1,2,3], [4,5,6]] d) [[-1,0,1], [2,3,-2], [4,1,1]] The correct answer is c) [[1,2,3], [4,5,6]]: “A square matrix has the same number of rows and columns.” The matrices in choices a), b), and d) are all square, but the matrices in choice c) are not square. ” .
- If A is a 3×4 matrix and B is a 4×2 matrix, what is the size of the product AB? a) 3×2 b) 3×4 c) 4×2 d) 3×3 Answer: a) 3×2 “The product of two matrices A and B is defined only if the number of columns in A is equal to the number of rows in B.” Let A and B each have 4 columns and 4 rows. Then, AB will have 3 columns and 2 rows. ” .
- a) [[1,0], [0,1]] b) [[2,0], [0,2]] c) [[1,2], [3,4]] d) [[0,0], [0,0]] The correct answer is b) [[2,0], [0,2]]. “A scalar matrix is a diagonal matrix where all the diagonal elements are equal.” In choice (b), the matrix is a scalar matrix with diagonal elements that are both 2. ” .
- a) [[1,2], [2,1]] b) [[1,2], [3,4]] c) [[1,0], [0,1]] d) [[0,1], [1,0]] The correct answer is a) [[1,2], [2,1]]: “A matrix is symmetric if it is equal to its transpose.” The matrix in (a) is symmetric because it is the same as its transpose. ” .
- Which of the following is not a type of matrix multiplication? a) Dot product b) Scalar multiplication c) Cross product d) Matrix product? The correct answer is c) Cross product. When you do a cross product, you multiply two vectors together. The other options are all types of matrix multiplication. ” .
- Which of these matrices is not invertible? a) [[1,2], [3,4]] b) [[1,0], [0,1]] c) [[2,4], [1,2]] d) [[1,1], [2,2]] Answer: a) Cross product “A matrix is invertible if its determinant is not equal to zero.” It can’t be turned into a square because its determinant is -2, which is not equal to zero. But the determinant of matrix b) is 1, the determinant of matrix c) is 0, and the determinant of matrix is 0. Therefore, matrices , c), and d) are not invertible. >” .
- a) [[1,2,3]] b) [[1], [2], [3]] c) [[1,2], [3,4]] d) [[1]] The correct answer is a) [[1,2,3]]: “A row matrix is a matrix with only one row.” The matrix in option a) is a row matrix. ” .
- a) [[1,2], [2,1]] b) [[0,1], [-1,0]] c) [[1,0,0], [0,1,0], [0,0,1]] d) [[-1,0], [0,-1]] The correct answer is b) [[0,1], [-1,0]. “A matrix is skew-symmetric if it is equal to the negative of its transpose.” This property is met by the matrix in option b), so it is skew-symmetric. ” .
- Which of the following is not true about the inverse of a matrix? a) The inverse of a matrix is unique b) If a matrix A is invertible, then so is its inverse c) The product of a matrix and its inverse is equal to the identity matrix d) If a matrix A is not invertible, then its inverse can be found by taking the reciprocal of each element This means that choice d) is not a property of the inverse of a matrix. ” .
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FAQ
What is an example of determination in an interview?
What are the 3 C’s of interview questions?
What are the social determinants of Health?
Material Well-Being The Social Gradient. Life expectancy is shorter and most diseases are more common further down the social ladder. Health policy must tackle the social and economic determinants of health. Unemployment. Job security increases health, well-being and job satisfaction.
Do screening questions cover social determinants?
While the screening questions cover most aspects of the determinants, there are areas (transport, earnings, and experiencing exclusion/discrimination) which largely were not covered by the screening instruments. The cross-walk does show a good deal of overlap, however, in how questions are framed to screen for different social determinants.
Why is screening for social determinants of Health important?
Patients’ social needs related to housing, food, safety, etc., can create significant obstacles to high-quality care and contribute to poor health. Screening for social determinants of health without first equipping the practice to address identified needs would be ineffective and unethical.
Are social determinants a business challenge?
From the employer’s perspective, social determinants represent a “profound business challenge,” one felt through increased health plan costs, productivity losses and diminished health equity. And for employees and their families, SDOH are a seemingly insurmountable barrier preventing them from achieving optimal health.