From Sudopedia, the free Sudoku reference guide

Hanidoku

From Sudopedia

Hanidoku is a Sudoku variant which uses hexagonal cells. The image shows a Hanidoku example. Every horizontal, inclining diagonal and descending diagonal line must contain a consecutive series of digits. Because these lines are different in size, they do not all contain digits 1 through 9. Some may contain digits 1 through 5, others may contain digits 5 through 9. This adds a level of difficulty that you will not encounter in a regular Sudoku. Fortunately, there are no boxes that you have to take into account.

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