## Notice

I am no longer posting new puzzles to this blog. For all of my Sudoku puzzles, old and new, please visit Sudoku in another section of this website. I will still create and offer new puzzles, in batches of a couple of hundred, once a week or so.

## Sudoku #33

Category: Sudoku
Wed, 27 Jul 2005, 08:44

For a change, here's a puzzle with mirror symmetry instead of rotational symmetry.

This begs the question: What makes up a good Sudoku puzzle? One aspect is the puzzles appearance. Symmetry may not be essential to a good puzzle, but it makes the puzzle more pleasing to the eye. There are several types of symmetry, with rotational symmetry the most common in published puzzles. But there's no reason you can't have mirror symmetry, with the axis of reflection diagonal, vertical, or horizontal. And some published puzzles have more than one degree of symmetry.

Puzzles can also vary in the number of cells with initial seed values. Some equate the number of starting values with the level of difficulty. But although a greater number of start values may make a puzzle easier, you can still have an easy puzzle with as few as 24 start values. (A related question is: Is there a minimum number of start values required for a puzzle to have a unique solution?)

Another way to look at a Sudoku puzzle is the distribution of start values. When solving a puzzle manually, I generally start with the groups with the most start values. But sometimes a puzzle has a very even distribution, with all groups having either two or three starting values. This can sometimes result in a more challenging puzzle, but perhaps an esthetically less interesting one.

On the other hand, puzzles with a wide variance in the number of start values can be interesting too. A group with a lot of start values may give you a bit of a head start, but the groups with few or even no seed values might give you some headaches. I tend to like posting puzzles with one or two empty groups. For standard puzzles with rotational symmetry, usually it's a middle group that ends up empty.

Anyways, have a go at todays puzzle. As I mentioned, it features mirror symmetry about a diagonal. It also features two empty groups, and no more than four seed values per group. If this puzzle is too easy for you, check back in a couple of days for a harder puzzle with mirror symmetry.

Hans

 9 7 2 6
 2 1 5 3
 2 4 9 6
 4 7 8
 1 3 7 8
 2 7 4
 5 9