Number of matrices with determinant value $0$ A $3 \times 3$ matrix is formed using the elements from the set $\{-1,0,1\}$. How many matrices will have determinant value $0$.
Let matrix is \begin{bmatrix}
p & q & r\\
x & y & z \\
a & b &c
\end{bmatrix}
So total matrices formed will be $3^9$ and determinant is given by $\Delta=pyc+rxb+qza-rya-qxc-pzb$
Making some combinations I deduced that determinant value will go from $-4$ to $4$ but what approach should I follow to get number of determinants having value $0$?
 A: Fun question! We'll start by reducing the determinant $\bmod 3$. The matrices of determinant $\pm 1 \bmod 3$ are exactly the matrices which, interpreted as having entries in the finite field $\mathbb{F}_3 = \{ 0, \pm 1 \}$, are invertible, so there are
$$|GL_3(\mathbb{F}_3)| = (3^3 - 1)(3^3 - 3)(3^3 - 3^2) = 11232$$
of them, and hence there are
$$|M_3(\mathbb{F}_3)| - |GL_3(\mathbb{F}_3)| = 3^9 - |GL_3(\mathbb{F}_3)| = 8451$$
matrices with determinant $0 \bmod 3$ (which agrees with J.G.'s count in the comments: $7875 + 2 \cdot 288$). So (assuming you're right that the determinant is in $[-4, 4]$, I haven't checked this) we've reduced the problem to counting the set $S$ of matrices with determinant $\pm 3$.
Let $H$ be the hyperoctahedral group $C_2 \wr S_3$ of $3 \times 3$ signed permutation matrices. $|H| = 48$ and $H$ acts freely from either the left or the right on $S$ (because $S$ consists of invertible matrices over $\mathbb{Q}$), so we can already show that $48$ divides $|S|$. Explicitly, allowing $H$ to act on the left amounts to giving ourselves the freedom to permute the rows and multiply any of them by $-1$, and similarly allowing $H$ to act on the right amounts to giving ourselves the freedom to permute the columns and multiply any of them by $-1$. The rest of the argument will proceed as follows:

*

*Identify the equivalence classes under the action of $G = H \times H$, with $H$ acting on both the right and the left, by finding a canonical form for the matrices in each equivalence class.


*Compute the size of the orbit of each canonical form by computing the size of its stabilizer.

Lemma 1: A matrix $X \in S$ can have at most one $0$ in any row or column.

Proof. If any row or column contains two $0$s then Laplace expansion along that row or column gives that the determinant is at most $2$ in absolute value. $\Box$

Lemma 2: A matrix $X \in S$ can have at most one row with no $0$s, and similarly for columns. Hence $X$ has at least two $0$s.

Proof. $\det(X) \equiv 1 \bmod 2$, so the rows and columns of $X$ must be linearly independent $\bmod 2$, and in particular distinct. $\Box$

Lemma 3: A matrix $X \in S$ must have exactly two $0$s.

Proof. If it has $3$ or more then they must be in distinct rows or columns by Lemma 1, and then the Laplace expansion shows that the determinant is at most $2$ in absolute value. $\Box$
Now it follows that one column must have the form $(\pm 1, \pm 1, \pm 1)$ and the other two must be permutations of $(\pm 1, \pm 1, 0)$ where the $0$s are in distinct places, and similarly for the rows (the signs are not necessarily the same here and below). By permuting rows and columns and changing their signs we can reduce to a matrix of the form
$$X = \left[ \begin{array}{cc} 1 & 1 & 0 \\ 1 & \pm 1 & 1 \\ 0 & 1 & \pm 1 \end{array} \right]$$
and now there are only $4$ cases to check determinants. Exactly one of them works, and we get that there is a single orbit, generated by
$$X = \left[ \begin{array}{cc} 1 & 1 & 0 \\ 1 & -1 & 1 \\ 0 & 1 & 1 \end{array} \right].$$
At this point, we know not only that $48$ divides $|S|$ but that $|S|$ divides $|G| = |H \times H| = 48^2$. It remains to compute the size of the stabilizer $G_X$ of this matrix under the action of $H \times H$, and then we'll have that $|S| = \frac{|G|}{|G_X|} = \frac{48^2}{|G_X|}$ (by the orbit-stabilizer theorem).
We can compute this stabilizer as follows. First let's ignore signs and only consider the effect of permuting columns and rows. The second row and column are unique because they're the only ones that contain three nonzero entries, so we can only swap the first and third row, and the first and third column, and then it's not hard to see that the only permutation that works is to simultaneously swap the first and third row and the first and third column; in other words, to conjugate by the permutation $(13)$.
Next, let's consider the effect of signs. By conjugating by $(13)$ if necessary we can suppose that we're considering only the effect of a bunch of sign changes. To preserve $X$ each entry must be flipped in sign an even number of times, and working through what that implies about which rows and columns can have their signs flipped we get that every row and every column must have their sign flipped the same number of times. The unique non-identity element which does this flips the sign of every row and every column simultaneously; this is the central element $(-1, -1) \in H \times H$.
It follows that the stabilizer is $C_2 \times C_2$ and hence that
$$|S| = \frac{|H \times H|}{|C_2 \times C_2|} = \frac{48^2}{2^2} = 24^2 = 576$$
which agrees with the Python-generated answers in the comments. Or rather, strictly speaking we were supposed to calculate the number of matrices with determinant $0$, which is
$$8451 - 576 = \boxed{ 7875 }.$$
Probably a somewhat more geometric approach is possible; note that the problem can be interpreted as being about volumes of certain tetrahedra made of lattice points in $\mathbb{Z}^3$ with entries in $\{ 0, \pm 1 \}$, which form a $3 \times 3 \times 3$ cube.

The problem asks us to count the number of degenerate tetrahedra with the center as a vertex (and an ordering of the other $3$ vertices) and we reduced the problem in the first step to counting tetrahedra with volume $\frac{3}{6} = \frac{1}{2}$ (or something like that). The hyperoctahedral group $H$ then appears naturally as the symmetry group of this cube, although it's a bit less clear how to see the action of the second copy of $H$.
A: Forgive me for writing a second answer but math.SE gets very laggy when you try to make a single answer too long.
We can indeed solve the problem by counting lattice points. Let $C = \{ -1, 0, 1 \}^3 \subsetneq \mathbb{Z}^3$ be the cube I mentioned; we want to count the number of ordered triples of points $v_1, v_2, v_3$ in $C$, not necessarily distinct, which are linearly dependent (over $\mathbb{R}$ or equivalently $\mathbb{Z}$), or equivalently such that $\dim \text{span}(v_1, v_2, v_3) \le 2$. We'll do this by considering each possible dimension of the span in turn.
Dimension 0: This is easy, $v_1 = v_2 = v_3 = 0$ is the only possibility so there is $\boxed{1}$ triple in this case.
Dimension 1: Every line passing through the origin and a nonzero point in $C$ passes through exactly two nonzero points, some point $v$ and its negative $-v$, so there are $\frac{3^3 - 1}{2} = 13$ such lines. (Note that this matches up with the number of points in the projective plane $|\mathbb{P}^2(\mathbb{F}_3)| = 1 + 3 + 3^2$ as expected.)
Among the $3$ points $\{ -v, 0, v \}$ on such a line there are $3^3$ triples of points and the only one that doesn't span the line is $0, 0, 0$, so there are $3^3 - 1 = 26$ triples that do span the line, giving
$$13 \cdot 26 = \boxed{338}$$
triples in the dimension $1$ case.
Dimension 2: This is the tough one. I missed a few cases here for quite awhile. It turns out that there are $25$ different planes (passing through the origin) spanned by the points of $C$ (I thought that there were $13$ for hours, then $21$ briefly). My mistake was assuming that every plane passing through the points of $C$ could be described as the plane normal to another vector in $C$ (this is only true $\bmod 3$). I'll just state how it goes for now: there are

*

*$9$ planes passing through $9$ points (including the origin), arranged in a square or rectangle,

*$4$ planes passing through $7$ points, arranged in a hexagon, and

*$12$ planes passing through $5$ points, arranged in a rhombus (these are the ones I missed).

Among the $n$ points on a plane there are $n^3$ triples and the ones that span the plane are the ones that aren't either $0, 0, 0$ or the ones that span one of the $\frac{n-1}{2}$ lines. As in the count above, for each line there are $26$ triples of points spanning it, so in total there are
$$n^3 - 13(n-1) - 1$$
triples of points spanning an $n$-point plane. This gives
$$9 \cdot 624 + 4 \cdot 264 + 12 \cdot 72 = \boxed{7536}$$
triples in the dimension 2 case, and hence
$$1 + 338 + 7536 = \boxed{7875}$$
triples total, in agreement with my other answer and the Python scripts (although jeez it took awhile).
