The maximum value of the determinant among all 2×2 real symmetri...
Using maxima and minima of a function of two variables, we have f is maximum at
x = 0, y = 7 and therefore, maximum value of the determinant is 49
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The maximum value of the determinant among all 2×2 real symmetri...
$\times 2$ matrices with entries from $\{-1, 0, 1\}$ is $4$.
Proof: Let $A = \begin{pmatrix} a & b \\ c & d \end{pmatrix}$ be a $2 \times 2$ matrix with entries from $\{-1, 0, 1\}$. Then we have four cases:
Case 1: $a = b = c = d = 0$. In this case, $\det A = 0$.
Case 2: Two of $a, b, c, d$ are $0$, and the other two are $\pm 1$. Without loss of generality, assume $a = b = 0$ and $c = 1$ and $d = -1$. Then $\det A = -1$.
Case 3: Three of $a, b, c, d$ are $0$, and the other one is $\pm 1$. Without loss of generality, assume $a = b = c = 0$ and $d = 1$. Then $\det A = 0$.
Case 4: Two of $a, b, c, d$ are $-1$ or $1$, and the other two are different from each other and from $\pm 1$. Without loss of generality, assume $a = b = 1$, $c = -1$, and $d = 0$. Then $\det A = 1$.
Therefore, the maximum value of the determinant among all $2 \times 2$ matrices with entries from $\{-1, 0, 1\}$ is $4$. This occurs when $A = \begin{pmatrix} 1 & 1 \\ 1 & 0 \end{pmatrix}$.