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NoticeI 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 #13Category: Sudoku When I was young, I was told that something bad often happens to kids when they're 13. For instance, some kids break a leg. Whatever it was, it was because 13 was such an unlucky number. One friend of mine, a rabid spaceflight fan, celebrated his 13th birthday on April 13, 1970. Doubly unlucky, you might think. But even more so if you remember what else happened on that date. The Apollo 13 spacecraft carrying three astronauts was about 330,000km from Earth, rapidly approaching the Moon, when an oxygen tank exploded, crippling the spacecraft. Can you get any more unlucky than that? Through a lot of hard work by the three astronauts, as well as hundreds of people in Mission Control, the three managed a safe return to Earth using their Lunar Module as a "lifeboat". But the number 13 is also considered "lucky". "Lucky numbers" are constructed using a method similar to the Sieve of Erastosthenes, which can be used to construct prime numbers. First, from the sequence of positive integers, strike out every second number, leaving the sequence of odd numbers: 1, 3, 5, 7, 9, ... After 2, the next number left in the sequence is 3, so strike out every third number, resulting in the sequence: 1, 3, 7, 9, 13, 15, 19, ... The next number remaining is 7, and so strike out every seventh number. And so on. You are then left with the "Lucky Numbers": 1, 3, 7, 9, 13, 15, 21, 25, 31, 33, 37, 43, 49, 51, ... Anyways, I hope you feel lucky with today's challenging Sudoku puzzle. Hans
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