In this post, 3-fills leave a near solution in the bypass, with an entirely blank box. Box marking resolves it systematically.
Seeing the four 5-fills and box corner candidates, you might suspect that this is a follow up on the 4-star of the last post. But really, you should ignore the 5-fills until you’ve rung out the 3-fills.
That goes swiftly with both 3-fills resolved and the bypass ending in a near collapse. Except for the Center box.
The bypass trace shows what it takes to get there.
Box marking does the walls and the C box. Whenever you get slinks of more than a few values in a unit, start counting values and covering cells. You may spot a subset. Here, 8 values slink into 8 C cells, a naked octuplet. But that also means 3, the missing value, must be placed in r6c6.
Everything then falls into place.
Next week, we conclude this series of Dave Green basic level Sudoku with the Sunday 5-star of 10/3/21, which illustrates the essential facts about hidden subsets. Try it out beforehand.
Systematic Sudoku 