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Christmas puzzle #1: Christmas tree sudoku

Here at Chalkdust, we like to celebrate Christmas as much as the next magazine for the mathematically curious, and what better way to celebrate than with a few yuletide mathematical puzzles. We have three for you, the first one you can find below, the second one will be published tomorrow (Christmas Day), and the final one the day after (Boxing Day). They are the perfect accompaniment to an warming hot chocolate and mince pie. Each puzzle is related to the previous one, so keep a hold of your solutions ready for the next day. We hope you enjoy giving them a go and the whole team wishes you a very merry Christmas!

The rules

  • Normal sudoku rules apply: you must complete the 9×9 grid with the digits 1 to 9 such that each digit appears exactly once in each row, column, and 3×3 block.
  • The digits that appear on each thermometer must strictly increase as you move away from the bulb. The colours of the thermometers are purely decorative and do not affect the puzzle.
  • The digits on the baubles are all even.
  • The digits on the stars are all prime.

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Puzzles on square grids

Sudoku puzzles are incredibly popular with mathematicians and non-mathematicians alike. However once you have solved a few of them, or even written a program to solve them for you, then can become tedious.
But this does not mean an end to writing digits in boxes: as this blog post will reveal, there are a great number of other puzzles which revolve around arranging digits correctly.
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