## A Challenging Exercise in Sysudoku Basic

This post begins a review of Will Shortz’ Hard Sudoku, v.1.  The review details the solution of a pre-selected 10 puzzles from the “Beware! Very Challenging” section of that book. Nearly all Shortz puzzles reviewed in Sysudoku are solved in the Sysudoku Basic stage, and Hard Sudoku is consistent with that.  The review therefore offers participants an opportunity to observe the efficiency and entertainment inherent in Sysudoku Basic.

If you haven’t attempted Beware 105, or have solved it in a different manner, make a copy from the previous post and walk through each move in the trace, accounting for exactly where and exactly why it is made. You may have to go back to the Traces page on the menu bar to resolve some questions, but the essential facts are:

• that effects are indented in the line below their causes, and
• that all effects of a cause are posted on the grid before the next effect to the right on the list becomes a cause.

Do participate. It will be easy for most, but hard for experts. To gain any perspective on Sysudoku basic, they will have to suspend their own preferences. This means dealing with    boxes named as compass points, explicit strong links, calling them “slinks”, and limiting objectives to the stage at hand. In the bypass (one list) you’re only getting clues and candidates of subsets. In box marking (one list per value), you’re denoting the remaining strong links of each value. In Sysudoku grids, box defined slink candidates are at the top, row slink candidates in the southwest corner, and column slink candidates in the southeast corner. In line marking, you’re filling in all other candidates, with those remaining unslinked listed from the middle left side.

Each review post starts with the basic trace, and one or more grids at points of interest. The commentary will be about the advantages of Sysudoku basic that are illustrated in the review puzzles.

As to the relative difficulty of the Hard Sudoku Beware collection, it is basic level.

Ready? Copy of Beware 105 in hand? Here is the basic trace.

The bypass has three unresolved 3-fills.  A modest result.

Beware 105 has two naked pairs, and a near collapse in box marking.

The final collapse starts in line marking on the sixth line, getting to a line of five unfilled cells. The cause is a naked quad.

Compare the compactness and relevant detail of this trace with any other Sudoku trace you have actually tried to read. The reason it is possible is that you, the reader, provide so much of the detail. You have the currently updated grid to specify the effect. The why you can figure out.

If I’ve made a mistake or missed a move, you can write me a comment for a correction. Believe me, I use traces to correct oversights all the time.

Here is the Sysudoku grid as the quad is posted.

The sequence of line markings is reported in the trace. Of course 7 is a hidden single as well.

Next we checkpoint your Sysudoku basic on Hard Beware 115.

The post will also report on the new Guide page on pattern(template) analysis.