1.6 Why are certain repetitive activities more pleasurable than others? 15 1.6 Why are certain repetitive activities more pleasurable than others? Ivan crossed it all out and decided to begin right off with something very strong, in order to attract the reader’s attention at once, so he wrote that a cat had got on a tram-car, and then went back to the episode with the severed head. Michael Bulgakov, The Master and Margarita Let us turn our attention to the emotional side of mathematics, more specifically, to the personal psychological experience of people working with mathematical algorithms and routines. I wish to formulate here some of my observations and conjec- tures which may appear to be bizarre and out of tune from the usual discourse on mathematics. However, I tested some of them in a warm-up talk that I gave at the forum discussion Where do math- ematicians come from? [457], part of a very peculiar conference, that of the World Federation of National Mathematics Competi- tions (WFNCM). It was held in July 2006 in Cambridge, England. On my way from Manchester to Cambridge, four hours by train, I had seen three people solving Sudoku puzzles. In one case, a lady of middle age shared a table with me and I had a chance to watch, in all detail and with a growing fascination, how she was solving an elementary level Sudoku puzzle. Her actions followed a certain rhythm: first she inspected the puzzle row by row and column by column until she located a critical cell (whose value had been al- ready uniquely determined by the already known values in other cells) and then, with obvious agitation, checked that was indeed the case, happily wrote the digit in, smiled with a childish satisfac- tion, relaxed for a few seconds, and, after a short pause, started the search again. The next day, in my talk at the conference, I pointed out that, from a mathematical point of view, solving an elementary level Su- doku puzzle is nothing more than solving a triangular system of Boolean equations by back substitution, something very similar to what we do after a Gauss–Jordan elimination in a system of si- multaneous linear equations. But has anyone ever seen people on a train solving systems of linear equations from a newspaper?9 Why is Sudoku popular, when systems of linear equations are not? (Actually, I was slightly wrong: at the time of my talk, I was unaware of Kakuro, which combines linear and Boolean equations. But one still has to see whether Kakuro beats Sudoku in popular- ity.)

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