Let $A_{N{\times}N}$ be an $N{\times}N$ matrix and $\mathcal{S_{k}}$ be a subset of elements in $A$ such that exactly $k$ elements from every row and column in $A$ are in $\mathcal{S_{k}}$. Thus, $\mathcal{S_k}$ has cardinality $N{\cdot}k$, with $k \in \{1,2,..,N\}$.

\begin{equation*} A_{N,N} = \begin{pmatrix} a_{1,1} & a_{1,2} & \cdots & a_{1,N} \\ a_{2,1} & a_{2,2} & \cdots & a_{2,N} \\ \vdots & \vdots & \ddots & \vdots \\ a_{N,1} & a_{N,2} & \cdots & a_{N,N} \end{pmatrix} \end{equation*}

For instance, consider $A_{8{\times}8}$ as described below. Let subset $S_{2}$ of the matrix $A_{8{\times}8}$ be given by the elements in bold in $A_{8{\times}8}$. Note that $S$ can be any subset having as its elements exactly $k=2$ elements per column and row of $A_{8{\times}8}$.

\begin{equation*} A_{8,8} = \begin{pmatrix} \mathbf{a_{1,1}} & a_{1,2} & a_{1,3} & a_{1,4} & a_{1,5} & a_{1,6} & a_{1,7} & \mathbf{a_{1,8}} \\ \mathbf{a_{2,1}} & \mathbf{a_{2,2}} & a_{2,3} & a_{2,4} & a_{2,5} & a_{2,6} & a_{2,7} & a_{2,8} \\ a_{3,1} & \mathbf{a_{3,2}} & \mathbf{a_{3,3}} & a_{3,4} & a_{3,5} & a_{3,6} & a_{3,7} & a_{3,8} \\ a_{4,1} & a_{4,2} & \mathbf{a_{4,3}} & \mathbf{a_{4,4}} & a_{4,5} & a_{4,6} & a_{4,7} & a_{4,8} \\ a_{5,1} & a_{5,2} & a_{5,3} & \mathbf{a_{5,4}} & \mathbf{a_{5,5}} & a_{5,6} & a_{5,7} & a_{5,8} \\ a_{6,1} & a_{6,2} & a_{6,3} & a_{6,4} & \mathbf{a_{6,5}} & \mathbf{a_{6,6}} & a_{6,7} & a_{6,8} \\ a_{7,1} & a_{7,2} & a_{7,3} & a_{7,4} & a_{7,5} & \mathbf{a_{7,6}} & \mathbf{a_{7,7}} & a_{7,8} \\ a_{8,1} & a_{8,2} & a_{8,3} & a_{8,4} & a_{8,5} & a_{8,6} & \mathbf{a_{8,7}} & \mathbf{a_{8,8}} \end{pmatrix} \end{equation*}

Now, select randomly $m$ elements from $\mathcal{S_{k}}$ with replacement. Then, we create an induced matrix with just the rows and columns of $A_{N{\times}N}$ corresponding to the selected elements from $\mathcal{S_{k}}$. For instance, if after selecting $m$ elements with replacement, the uniquely selected elements from $S_{2}$ are the following 5 elements: $a_{1,1},a_{3,2},a_{3,3},a_{6,6},a_{8,8}$, then the resultant matrix is

\begin{equation*} A_{r} = \begin{pmatrix} \mathbf{a_{1,1}} & a_{1,2} & a_{1,3} & a_{1,6} & \mathbf{a_{1,8}} \\ a_{3,1} & \mathbf{a_{3,2}} & \mathbf{a_{3,3}} & a_{3,6} & a_{3,8} \\ a_{6,1} & a_{6,2} & a_{6,3} & \mathbf{a_{6,6}} & a_{6,8} \\ a_{8,1} & a_{8,2} & a_{8,3} & a_{8,6} & \mathbf{a_{8,8}} \end{pmatrix} \end{equation*}

Let $X$ be the number of elements in $\mathcal{S_{k}}$ of the resultant matrix ($A_{r}$). For the given example, $x=6$.

My Question: How we can calculate the average of $X$ for given $m,N,k$ ($E[X](N,k,m))$?

I have already calculated the average number of columns $E(C)$ when $m$ elements are randomly selected with replacement from $\mathcal{S_{k}}$. Note that for this case $E(C)$ is equal to the average number of rows $E(R)$ and can be calculated as:

$E[C](N,m)=N*P_{chosen}$, where $P_{chosen}=1-(1-(1/N))^m)$ is the probability that a column of the original matrix is selected at least once. Thus, $A_{r}$ is a $E[C] \times E[R]$ matrix.

Thank you for any help!

An alternative phrasing: Write $I_N = \{1, 2, \cdots, N\}$. Let $S \in I_N \times I_N$ be a subset such that $|S \cap (\{i\} \times I_N)| = |S \cap (I_N \times \{i\})| = 2$. Note that $|S| = 2N$.

Then for given $0 \leq m \leq 2k$, what is the distribution of $|p_1(M)| |p_2(M_m)|$, where $M_m$ ranges uniformly over the set of $m$-element subsets of $S$, and $p_1, p_2$ are the projection functions?

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    $\begingroup$ I don't quite understand this question—for example, is it a question about a particular matrix $A$, or about any matrix?—but this seems to be an elementary probability question, not a research-level question, and so does not belong on MO. If you want to clarify it for MO, or ask it again on MSE, I encourage you to TeX it so that it will be easier to read. $\endgroup$
    – LSpice
    Commented May 10, 2020 at 0:43
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    $\begingroup$ Welcome to MO. It's good to have an example, but eventually the question is really unclear. It might be that the question would eventually fit MO, but I currently can't quite follow. Use crisper definitions and state a more precise question. $\endgroup$
    – Amir Sagiv
    Commented May 10, 2020 at 0:46
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    $\begingroup$ Thank you @LSpice for your comments and suggestions. I rephrased the problem, detailed it better, and used LaTex. I don't know if it fits better here or on MSE. $\endgroup$ Commented May 10, 2020 at 2:10
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    $\begingroup$ Thank you @AmirSagiv. I tried to explain better the problem. Please help me to define if it fits better here or on MSE. $\endgroup$ Commented May 10, 2020 at 2:11
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    $\begingroup$ It seems to me that it would be clearer to phrase this without the matrix. $\endgroup$
    – user44191
    Commented May 10, 2020 at 14:57

1 Answer 1


Suppose that we selected $m$ random elements of $S_k$. An element $s$ of $S_k$ appears in the induced matrix iff (i) there is a selected element in the row of $s$ in $A$; and (ii) there is a selected element in the column of $s$ in $A$. Call such an element $s$ lucky, and so $X$ is the number of lucky elements.

Under selection without replacement, the probability $P$ of a fixed element $s\in S_k$ to be lucky equals $$P = 1 - \frac{2\binom{Nk-k}{m} - \binom{Nk-(2k-1)}{m}}{\binom{Nk}{m}},$$ where $\binom{Nk-k}{m}/\binom{Nk}{m}$ is the probability that nothing is selected from the row of $s$ in $A$, and $\binom{Nk-(2k-1)}{m}/\binom{Nk}{m}$ is the probability that nothing is selected from neither the row nor the column of $s$ in $A$.

Similarly, under selection with replacement, we have $$P = 1 - \frac{2(Nk-k)^m - (Nk-(2k-1))^m}{(Nk)^m}.$$

Then $$E[X](m,N,k) = Nk\cdot P.$$

  • $\begingroup$ Thank you @Max for your answer. It seems that in your solution $m$ is the number of unique selected elements, is it right? $\endgroup$ Commented May 10, 2020 at 13:27
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    $\begingroup$ @CarlosA.AstudilloTrujillo: Yes. Selection with replacement is similar - added it now $\endgroup$ Commented May 10, 2020 at 13:43
  • $\begingroup$ Another option to answer my initial question is just to consider $m$ as $m^{′}$ in your formula without replacement and calculate $m{′}=N{\cdot}k\left(1−B_{0}(n,p)\right)$, where $B_{r}(n,p)$ is the binomial distribution with parameters $n=m$ (as defined in the initial question), $p=\frac{1}{N{\cdot}k}$ and $r$ is the number of times an element in $S_k$ is selected. Thus, $1−B_{0}(n,p)$ is the probability that an element of $S_{k}$ is selected at least once in $n$ trials. $\endgroup$ Commented May 10, 2020 at 14:30
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    $\begingroup$ I tested your solution of selection with replacement and it is more accurate than my previous suggestion of considering $m^{'}$ and selection without replacement. Thank you again. $\endgroup$ Commented May 10, 2020 at 15:22

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