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Let $\mathbf{A}\in\left\{ 0,1\right\} ^{d\times d}$, and $k(\mathbf{A})$ be the minimal number of $1$s in any column or row in $\mathbf{A}$.

Question: What is the minimal $k$ such that, for any $\mathbf{A}$ with $k\left(\mathbf{A}\right)=k$, we can always find a permutation matrix, $\mathbf{B\in}\left\{ 0,1\right\} ^{d\times d}$, which $1$s only fall on the $1$s of $\mathbf{A}$, i.e., $\forall i,j:\,B_{ij}=B_{ij}A_{ij}$?

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You are asking for a matching in a bipartite graph. The two sides of the graph are the row indices $X=\{1,\dots,d\}$, and the column indices $Y=\{1,\dots,d\}$. The number $k$ is the maximal vertex degree (how many "valid" columns do we have in each row).

$k=\lceil d/2 \rceil$ is the amount needed by an application of Hall's Marriage Theorem. A bipartite graph on the parts $X,Y$ so that any subset $S \subset X$ has at least $|S|$ neighbors in $Y$ has an $X$-complete matching.

If $k \lt \lceil d/2 \rceil$, then consider the incidence matrix of the complete bipartite graph $K_{k,d-k}$. Since there are $d-k$ vertices connected to only $k$, there is no matching.

If $k \ge \lceil d/2 \rceil$, then any nonempty $S \subset X$ has at least $d/2$ neighbors $n(S)$, so if $|S| \gt |n(S)|$ then $|S| \gt d/2$. Since every vertex in $Y$ has degree at least $d/2$, by the pigeonhole principle it must be connected to some vertex in $S$, so $n(S)=Y$. That implies there is a matching.

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  • $\begingroup$ Many thanks! I missed this connection. $\endgroup$ Commented Jun 18, 2017 at 9:07
  • $\begingroup$ We do not need Hall's theorem when $k\geqslant \lceil d/2\rceil$. If $(a_i,b_i),i=1,\dots,k$ is a maximal matching and $a,b$ are the vertices from $A,B$ uncovered by it, then by pigeonhole principle there exists $i$ such that $a$ is joined with $b_i$ and $b$ with $a_i$, this allows to increase our matching. $\endgroup$ Commented Jun 18, 2017 at 9:38
  • $\begingroup$ An example that is helpful for me is the matrix with last k rows and last k columns filled with 1's, and 0's in the upper left. If k is less than d/2, this matrix contains no permutation matrix in your sense. Gerhard "Or In My Sense Either" Paseman, 2017.06.18. $\endgroup$ Commented Jun 18, 2017 at 18:46

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