I agree with Tom, with the 3rd constraint. If I take part in a sudoku tournament and there are only latin squares, I'll think it's not a sudoku tournament !

For me it's even more than a 3rd constraint; it must be a "local constraint" (as explained Tom):

Some sudoku can have more than 2 directional constraints, for example star sudoku (see

http://www.sachsentext.de/gif/starsudoku1.gif for example). There are 3 directions + 1 box constraint. From my point of view, if there were not box constraint, it should not be called "sudoku" (but... sort of... "latin star"?) even if there are 3 (directional) constraint.

ronaldx wrote:- so that one such symbol appears in each cell,

- certain given subsets of the cells must contain each symbol exactly once.

- What about: sudoku 0-9 (http://www.sachsentext.de/gif/sudok09b.gif) or tight fit sudoku (http://www.stanford.edu/~tsnyder/tightfit2.jpg) ? it must have more than one such symbol in some cells.
- I've seen variants of sudoku, for example subsets3*3 on argio-logic.net (Italian site), where each symbol appears more than one in every subsets (in this example you have to fill grids with digits 1, 2 and 3 so that each digit appears exactly 3 times in every row, column and box. Can we call this sudoku or not?

ronaldx wrote:2) No (an additional non-sudoku rule is required; though I'd agree it's a sudoku-variant, it's also a skyscrapers-variant)

If you presented a lay-person with a puzzle and gave no rules but "this is a sudoku", would they immediately understand what they were expected to do?

With a 7x7 latin square - I think the answer is yes.

In my opinion a sudoku-variant can be called sudoku. Of course, to solve this one, you have to know the meaning of outside clues. But it is sufficiant to explain the meaning of these and say that sudoku rules apply. If you present a 9*9 latin square saying only "this is a sudoku", I think someone can think that people who make this grid forget to design 3*3 boxes and try to solve using this constraint, even if boxes are not apparent... Of course with 7*7 example, it's not the same because 7*7 sudoku are less common.