Wolves and Sheep, with tables

Previously on this blog: “Wolves and sheep” (2019-04-17). Since April, I’ve learned that this puzzle is exactly equivalent to “What is the smallest number \(t\) of rows that a \(n\)-column matrix can have and remain \(d\)-separable?” As with the “Discrete Peaceful Encampments” puzzle (2019-10-18), the answers to this question can be tabulated as a triangular matrix:

```
d= 1 2 3 4 5 6 ...
.
n=1 . 0
n=2 . 1 0
n=3 2 2 0
n=4 2 3 3 0
n=5 3 4 4 4 0
n=6 3 5 5 5 5 0
n=7 3 6 6 6 6 6 0
n=8 3 6 7 7 7 7 7 0
n=9 4 7 8 8 8 8 8 8 0
n=10 4 7 9 9 9 9 9 9 9 0
n=11 4 8 10 10 10 10 10 10 10 10 0
n=12 4 8 11 11 11 11 11 11 11 11 11 0
n=13 4 8 12 12 12 12 12 12 12 12 12 12 0
n=14 4 9 12 13 13 13 13 13 13 13 13 13 13 0
n=15 4 9 13 14 14 14 14 14 14 14 14 14 14 14 0
n=16 4 9 . . 15 15 15 15 15 15 15 15 15 15 15 0
n=17 5 9 . . 16 16 16 16 16 16 16 16 16 16 16 16 0
n=18 5 10 . . . 17 17 17 17 17 17 17 17 17 17 17 17 0
n=19 5 10 . . . 18 18 18 18 18 18 18 18 18 18 18 18 18 0
n=20 5 10 . . . . 19 19 19 19 19 19 19 19 19 19 19 19 19 0
```

UPDATE, 2023-03-31: The non-zero elements of this triangle are now OEIS sequence A361928.

Column \(d=1\) is simply \(\lceil\lg{n\choose k}\rceil\).

Column \(d=2\) is related to OEIS sequence A054961, the maximal number of columns \(n\) in a 2-separable matrix with \(t\) rows. Dmitry Kamenetsky has determined that \(t(22, 2)\le 10\), \(t(31, 2)\le 11\), and \(t(46, 2)\le 12\). Zhao Hui Du adds that \(t(55, 2)\le 13\), \(t(63, 2)\le 14\), etc.; see the OEIS for more details.

Column \(d=3\) is related to OEIS sequence A290492, the maximal number of columns \(n\) in a 3-separable matrix with \(t\) rows.

Many, many thanks to Elaqqad over on Math.SE for helping me come to these realizations, and for providing sample solutions for \(t(111,3)\le 37\) and \(t(104,5)\le 59\).

As with “Discrete Peaceful Encampments,” I provide a text file of best known solutions for many different \(n\) and \(d\) (not just the integer \(t(n,d)\) for each pair, but an actual \(t\times n\) matrix which is verifiably \(d\)-separable). You can find the current file here.

Unlike “Discrete Peaceful Encampments,” *many* of the solutions to “Wolves and Sheep” are trivially
derived from the solutions to “adjacent” instances of the problem. For example, the solution for
\(t(n, d)\) often looks very much like the solution for \(t(n+1, d)\) with one column missing;
and the solutions for \(t(n, 1)\) all follow a simple pattern. So the solution file contains only
those solutions which are “irreducible” — which can’t be expressed by simple manipulations of other
solutions. I wrote a simple program to read in the solution file, apply those simple manipulations,
and print out the best known solution for any \((n,d)\).

```
$ ./wolfy --verify 14 3
Candidate is
N=14 D=3 T=12 guaranteed_best=0
1.1.....1...1.
...1.1......11
.....11.11....
..1.11....1...
....1...1..1.1
.11......1...1
..11..1....1..
.1.1....1.1...
1.....1...1..1
1..11....1....
.........1111.
.1..1.1.....1.
Verified. This is a solution for t(14, 3) <= 12.
```

My file contains some very large instances for \(d=2\) developed by Zhao Hui Du; the largest at the moment is \(t(1090, 2)\le 27\). See his blog post “两瓶毒酒问题” (October 2019); the data file is found at the last link on that page (currently here).

I’d be interested to see some more examples of “irreducible” \(d\)-separable matrices for small \(n\). If you know where to find such examples, please send them my way! I’ll add them to my text file here.

Find further StackExchange discussions here, here, and here.