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  • What is the maximum possible number of entries of A that are even? What if A is a matrix of order n?

  • What is the maximum possible number of entries of A that are PRIMES? What if A is a matrix of order n?

This question was asked during my class test and I was unable to do this.

Please help me in doing this.

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    $\begingroup$ What set are the entries from? $\endgroup$
    – joriki
    Jun 3, 2016 at 5:49
  • $\begingroup$ @joriki:Integers $\endgroup$
    – Styles
    Aug 28, 2016 at 14:39

3 Answers 3

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For the first question, you can show that $I_3$ is an example of such a matrix with $6$ even entries. If there are $7$ or more event entries, then necessarily there is a row with only even entries, calculate your determinant by expanding this line, it is a sum of even terms, thus the determinant is even and can't be $1$.
Thus $6$ is the maximum possible number of entries of A that are even and such as $\det(A)=1$.


The generalisation for matrices of order $n$, for the first question is $n^2-n$, you can prove this with same reasoning that in the case $3 \times 3$ : $I_n$ has $n^2-n$ even entries and its determinant is $1$. And if your matrix has more that $n^2-n$ even entries then there is necessary a whole row with even entries and you can expand your determinant from this row and thus it will be even and so different of $1$.

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  • $\begingroup$ :But,its determinant is p,we've to consider those matrices whose determinant is 1 $\endgroup$
    – Styles
    Jun 3, 2016 at 6:12
  • $\begingroup$ Try $\begin{matrix} 1 & 2 & 2 \\ 2& 1&2 \\ 0 &2 &1 \end{matrix}$. $\endgroup$
    – Hetebrij
    Jun 3, 2016 at 6:22
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For first part answer is $6$. You cab take $A$ as $$A=\begin{bmatrix} 1&0&0\\b&a&ac-1\\d&1&c\end{bmatrix}$$ where $a,b,c,d$ are any even number. Now consider $$e_3=6,$$ $$e_4=e_3+2(4-1)$$ $$e_n=e_{n-1}+2(n-1)$$ where $e_n$ is no. of even entries when $A$ is of order $n\ge3$.

For second part take $A$ as $$A=\begin{bmatrix} 1&0&0\\a&p_1&p_2\\b&p_3&p_4\end{bmatrix}$$ where $a,b$ are any prime number and $p_1p_4-p_3p_2=1$ it is possible to find such primes e.g take $p_1=3,p_4=5,p_3=2,p_2=7$. Now consider $$p_3=6,$$ $$e_4=e_3+(4-1)$$ $$e_n=e_{n-1}+(n-1)$$ where $p_n$ is no. of prime entries when $A$ is of order $n\ge3$.

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    $\begingroup$ For the first part, $\frac{1}{ab}$ is not an integer. $\endgroup$
    – Hetebrij
    Jun 3, 2016 at 6:29
  • $\begingroup$ @Sry:How can we prove these recursion formulas. $\endgroup$
    – Styles
    Jun 3, 2016 at 7:00
  • $\begingroup$ Say we are going for matrix of order $4$, it will be of same format i.e first row has $a_{11}=1$ entry fixed so rest three are left for even choices again first column has 3 entries left for even choices. Apart from first row and first column, leftover matrix is of $3\times3$ whose even choice we know. Similarly if we go for $5\times5$ matrix First row and column has $4$ choices each for even entries and leftover $4\times4$ matrix has $e_4 $ choices we know already. So.... $\endgroup$
    – Sry
    Jun 3, 2016 at 7:34
  • $\begingroup$ So, what is the maximum possible number of entries of a $3 \times 3$ matrix $A$ that are primes? $\endgroup$ Dec 6, 2019 at 18:40
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To a matrix of order 3, maximum possibility of even entries is 6 ( take identity matrix of order 3) but as you put even entries on more than 6 places then you can find a row consisting of even entries along which you can take determinant.We can also generalize this for order n, may be possibility for maximum even entries is n^{2}-n( which is Identity matrix)

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  • $\begingroup$ see identity matrix is always a case $\endgroup$
    – Sunny
    Jun 3, 2016 at 6:10
  • $\begingroup$ There is no conviction in the argument $\endgroup$
    – Shailesh
    Jun 3, 2016 at 6:18

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