Intereting Posts

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Formula for midpoint in hyperbolic 3-space
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How to solve ODEs by converting it to Clairaut's form through suitable substitutions.
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Given a vector $\mathbf{x} \in \mathbb{R}^N$, let’s define:

$$\text{diag}(\mathbf{x}) = \begin{pmatrix}

x_1 & 0 & \ldots & 0 \\

0 & x_2 & \ldots & 0 \\

\vdots & \vdots & \ddots & \vdots \\

0 & 0 & \ldots & x_N

\end{pmatrix}.$$

Moreover, let

- Adjoint functors as “conceptual inverses”
- Inverse Laplace transform of one complicated function
- If $\arctan(x)+\arctan(y)+\arctan(z)=\pi/2$ how to show that $xy+yz+zx=1$?
- Does $(\mathbf A+\epsilon \mathbf I)^{-1}$ always exist? Why?
- Functions that are their own inversion.
- Why is the inverse of a sum of matrices not the sum of their inverses?

$$\mathbf{1}= \begin{pmatrix}

1 & 1 & \ldots & 1 \\

1 & 1 & \ldots & 1 \\

\vdots & \vdots & \ddots & \vdots \\

1 & 1 & \ldots & 1

\end{pmatrix}.$$

Here is my question:

When is the matrix $\mathbf{M} = \text{diag}(\mathbf{x}) + \mathbf{1}$ invertible?

I was able to find some results when $x_1 = x_2 = \ldots = x_N = x$. Indeed, the matrix $M$ is singular when:

- $x=0$. This is trivial since $\mathbf{M} = \mathbf{1}$…
- $x=-N$. In this case, if you sum up all the rows (or columns) of the matrix $M$, you get the zero vector.

What can I say in the general case when $\text{diag}(\mathbf{x})$ is a generic vector?

- positive definite quadratic form
- Nilpotent matrix and basis for $F^n$
- For a matrix $A$, is $\|A\| \leq {\lambda}^{1/2}$ true?
- Origin and use of an identity of formal power series: $\det(1 - \psi T) = \exp \left(-\sum_{s=1}^{\infty} \text{Tr}(\psi^{s})T^{s}/s\right)$
- How to visualize a rank-2 tensor?
- Importance of Linear Algebra
- On the change of coordinates of a matrix.
- Determining whether a symmetric matrix is positive-definite (algorithm)
- Forecast equivalence between two steady-state Kalman filters
- Sum of the entries in the matrix $A^3$

We can easily compute the determinant of the sum $\operatorname{diag}(\mathbf{x}) + \mathbf{1}$ and check invertibility that way,

If all the diagonal entries $x_i$ are nonzero, we can apply the matrix determinant lemma for a rank one update to an invertible matrix:

$$ \det(A+uv^T) = (1 + v^T A^{-1} u) \det(A) $$

When $A$ is the matrix $\operatorname{diag}(\mathbf{x})$ and $u,v$ are vectors of all ones, this says the matrix sum is invertible unless the sum of the reciprocals $x_i^{-1}$ is $-1$.

If one of the diagonal entries is zero, say $x_1$ without loss of generality, then elementary row operations quickly show that the determinant of $\operatorname{diag}(\mathbf{x}) + \mathbf{1}$ is $\prod_{k=2}^n x_k$.

i think your matrix is nonsingular iff

$$1 +\frac 1{x_1}+\frac 1{x_2}+ \ldots + \frac 1{x_n} \ne 0$$

i will look at the case $n = 4$ will consider the matrix $A = D + jj^\top$ where $D$ is the diagonal matrix with entries $d_1, d_2, d_3$ and $d_4, j = (1,1,1,1)^\top.$

suppose $\lambda, x$ is an eigenvalue-eigenvector pair. then we have

$$d_1x_1 + x_1 + x_2+ x_3 + x_4 = \lambda x_1, \ldots, x_1+x_2+x_3+x_4 + d_4x_4=\lambda x_4 $$ solving for $x_1$ we find that

$$x_1= \frac 1{(\lambda- d_1)}(x_1+x_2+x_3+x_4), \ldots

x_4= \frac 1{(\lambda- d_4)}\left(x_1+x_2+x_3+x_4\right)\tag 1 $$ adding the four equations in (1), you find that the characteristic equation of $A$ is

$$1 = \frac 1{(\lambda-d_1)}+\frac 1{(\lambda-d_2)}+\frac 1{(\lambda-d_3)}+\frac 1{(\lambda-d_4)} $$

therefore the matrix $D + jj^\top$ is singular iff $$ \frac 1d_1 + \frac 1d_2 + \frac 1d_3 + \frac 1d_4 + 1 \ne 0. $$

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- How to prove that there exist infinitely many integer solutions to the equation $x^2-ny^2=1$ without Algebraic Number Theory
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- Intersection of Connected Sets
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- Is the Riemann integral of a strictly positive function positive?
- Why is the following language decidable? $L_{one\ right\ and\ never\ stops}$