If $A$ with a size of $n\times 200$, does the inverse of $AA^T$ always exist?
86919
When (and more importantly, why) does $AA^T$ have an inverse?
21782
Is $\aleph_0 = \mathbb{N}$?
14161
Proof of when is $A=X^TX$ invertible?
96995
Is the inverse of a linear transformation linear as well?
61274
If $AA^T$ is the zero matrix, then $A$ is the zero matrix
103695
Prove Inverses not need be Unique if Associativity fails
62067
Inverse function of $x^x$
20161
Prove that the multiplicative inverse of $a$ modulo $m$ exists if and only if $a$ and $m$ are coprime.
22890
Is it coherent to extend $\mathbb{R}$ with a reciprocal of $0$?
67784
If $A + A^t = 2I$ then $\det(A) \geq 1$
153290
$A^TA=B^TB$. Is $A=QB$ for some orthogonal $Q$?
79011
If $(F:E)<\infty$, is it always true that $\operatorname{Aut}(F/E)\leq(F:E)?$
82097
If $A \succeq B$, is it true that $B^{-1} \succeq A^{-1}$?
25159
Why does the inverse of the Hilbert matrix have integer entries?
94855
Prove if a (mod n) has a multiplicative inverse, then it's unique
152665
$A^TA+I$ is always invertible?
141286
When is $\mathbf{X}^{T}\mathbf{X}+\lambda\mathbf{I}$ invertible?