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Is there any way to solve hard Sudoku (with 17-19 digits or so) without any pencil marks? What is/are the way(s) of finding a make single without a pencil mark? Is there any book on it?

Hi Dipro,

Dipro wrote:Is there any way to solve hard Sudoku (with 17-19 digits or so) without any pencil marks? What is/are the way(s) of finding a make single without a pencil mark? Is there any book on it?

I don't think you can solve difficult puzzles without pencil marking, unless you have an outstanding memory. I know of a book whose author, a serious mathematician, gives the keys to solving without marking, but I remain doubtful about it.
The book is in French, here is a link to Amazon to find it:
https://www.cultura.com/cinq-techniques ... 50561.html
Cordialy
Robert