## All the Way With Dave

This post goes through Sysudoku Basic with the Dave Green puzzle published in the Akron Beacon Journal on Sunday October 10, 2021. It’s the first in a series of published Conceptis puzzles selected to illustrate basic moves introduced in the later years of Systematic Sudoku, the blog.  In this series, we pause before many significant moves, to give the trace readers the opportunity to see if the move is in their spotting repertoires, and if they know how and why the move works.

Starting on the bypass, is your first move C1? It takes a dublex in the middle colulmns to limit C1 to c5 and a cross-hatch in box E to limit it to r4. That move shows the satisfying challenge of the bypass. It also shows the value of the uncluttered grid as you pit clues and subsets against clues and subsets.

After (C1, W2, NE3), column c7 is a 4-fill. You respond by checking for missing values seeing more than 1 blank cell. In c7 missing values 2 and 6 both see two. SE6 fails to see a third, but 2 does, placing SE2, and leaving a 3-fill in c7.  NE3 also creates a 4-fill in r3. Will the 3-fill or 4-fill be resolved?

All that action can be recorded in the trace, which handles the bookkeeping, and keeps you apprised of exactly where you are. You read a trace by filling out your own grid as you go.

Tracing the fill of c7, the 3-fill does resolve, and its clue SE5 creates another 4-fill in r7. Two of the four missing values place themselves in separate boxes, leaving two naked pairs in the boxes.

Moving ahead, W3 closing a 2 by 2 square in the West box is the next  significant event.

Line c3 missing the square has value 6, not in the square. That places 6 in r4c12  E4, and a new 4-fill in r4.

Continuing through the bypass, the E box fills in, and two more 4-fills are resolved.

Here is the grid passed on to box marking.

Box marking brings in box slinks, marking them with “pencil marks” along the cell top. For trace readers, only the box need be named for each value. Here the combined marking for 1 and 2 in NE creates a naked pair, reserving two cells for a c8 3-fill, promptly resolved.

Continuing in box marking value by value. The 6 list adds the 6 value to the three unfilled cells of r6. That finishes a wall in the C box, starting a plunging follow up for you to complete and trace.

The grid passed on to line marking incudes another resolved 3-fill in the 7’s.

It’s all on the box marking trace:

Line marking completes the candidate field. Each row or each column is assigned a fill string of digits including all missing candidates on that line. The fill string is copied along the line, and digits seen by clues or subsets are deleted. The remaining candidates are placed in the cell, with slink partners positioned to identify the direction of the slink. Lines are filled in increasing order of free cells, so easier fills come first. The trace records the order of line filling.

Bv are marked and candidates are positioned as each line is filled. When rows or columns are marked , the unfilled lines in the other direction are marked. This is the close and it doesn’t require fill strings.

Here is the line marked grid. Just as the Close is started on c4, a hidden single 7 is discovered in the column being filled. The line marking trace showing the collapse and the solution follow.

The line marking trace on the rows is short here, because so many were already filled. The close finishes line marking on the columns, with the collapse on the first one.

This Sysudoku Basic series of Dave Green puzzles actually started last May 25, 2021 when two  Sunday 5-stars full of 3, 4, and yes, a 5-fill, were posted. You can look them up by post date on the monthly list on the right. That, and the titles page, is a ticket to hundreds of bypass traces.

Next time,  we watch a bypass starting with two 4-fills, and box marking get a 5-star to coloring. The conclusion is a surprise. See if you can DIY this one.