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Questions about the properties of vector spaces and linear transformations, including linear systems in general.

13 votes
1 answer
882 views

Probability that random nonnegative integer matrix is singular

Q. What is the probability that an $n \times n$ matrix, whose elements are independent uniformly random integers in $\{0,1,\ldots,k\}$, is singular? For example, for $n=3$ and $k=2$, the first …
Joseph O'Rourke's user avatar
10 votes
2 answers
1k views

Does the hyperdeterminant calculate a quantity akin to the volume of a parallelepiped?

If $M$ is an $n \times n$ matrix, $|\det(M)|$ is the volume of the $n$-dimensional parallelepiped spanned by the column vectors of $M$.                     (Image from Wikipedia's Determinant ar …
Joseph O'Rourke's user avatar
109 votes
15 answers
12k views

Why are matrices ubiquitous but hypermatrices rare?

I am puzzled by the amazing utility and therefore ubiquity of two-dimensional matrices in comparison to the relative paucity of multidimensional arrays of numbers, hypermatrices. Of course multidimens …
Joseph O'Rourke's user avatar
16 votes
4 answers
914 views

Integer matrices whose determinant equals their norm

Let $M$ be an $2 \times 2$ matrix, with all entries in $\mathbb{N}$: $$ M= \begin{bmatrix} a & b \\ c & d \end{bmatrix} \;. $$ So $$ \mathrm{det}(M) = a d - b c \; . $$ The Euclidean norm (a.k.a. F …
Joseph O'Rourke's user avatar
16 votes
1 answer
765 views

Minimizing the excursion of a sum of unit vectors

I have $n$ unit-length vectors $v_i$ in $\mathbb{R}^3$, whose sum is zero: $$ v_1 + v_2 + \cdots + v_n = 0 \; .$$ Now I form the closed polygon $P$ in space by placing them head to tail. So the vertic …
Joseph O'Rourke's user avatar