## Dave Green’s Three Column Bypass 3-Fill

This post traces the bypass solution of Dave Green’s Friday **** of July 7, 2017, an amazing follow up to my recent addition of 3-fill’s to the slink marking bypass. This puzzle, starting with  three column 3-fills among the givens, was displayed in the previous post.

Over the years, I’ve several times had the feeling that Dave Green, the Sudoku composer of record for my newspaper, is teasing me with puzzles illustrating themes of a recent posts. It’s more likely to be the posts alerting me to ideas in the puzzles.

But the latest episode with Green’s 3-fill is striking. Although the line marking I’ve advocated  since 2011 includes  3-fill lines, I’ve just very recently made the 3-fill rules a part of the slink marking bypass. It’s a conscious effort to enhance human solving by reducing clutter of unexploited pencil marks, and by utilizing the human ability to visualize aligned slinks and their effects without writing them down. So here is Green with a sumptuous platter of 3-fill lines, the likes of which I had never noticed in his column before. Why now?

Anyway, let’s be thankful. Here are two solution traces. The first gets to a bypass collapse point while going for the 3-fill from the beginning. Later we look at what happens when we hold out against the 3-fill until it becomes necessary.  I recommended the first solving path last post, starting with the 3-fill of c3.

Four lines down into the trace, all three 3-fill columns, plus two rows are filled, one as a 3-fill.

More 3-fill opportunities come up as clues are added.

With collapse eminent, the grid is loaded with leftover effects, the single pencil marks in cells,  that have not been tried as causes.  The puzzle has no chance.

Maybe the 3-fill process of identifying the missing numbers, and testing how many of them are seen from each 3-fill cell is not your preferred way of doing the bypass. If you prefer, you can call on the 3-fill only when needed.

To try that in this case, we place the 3-fill last in the effects lists, and start with the regular survey of increasing numbers. That works well, but when it stalls, the 3-fill keeps it going.

Next is a two-post review of Rebecca Bean’s 600 Extremely Hard Sudoku Puzzles with Answers. In this book, the puzzles and answers are put together in 12 sections of 50. For the review I pre-selected puzzle 6 in each section. “Extremely Hard ” isn’t the Sysudoku rating, and maybe you should estimate your own rating, so here are two puzzles that will be checkpointed in the next post, 7-6  and 12-6. The second post will checkpoint 9-6 and 11-6. Enjoy.