Here a Manuel Castillo Sudoku sends me directly to last resort city. Although the bv field eludes productive XY-chains or AIC, coloring is decisive enough to solve 347 in a single trial.
Your first view of the grid comes with a blue green cluster, and six AIC hinges I marked when I came up with nothing from the bv scan and X-panel.
If you’d like to work it from the beginning and didn’t get Castillo’s Only Extreme Sudoku yet, just load the clues and go for it. The grid is in the line marked state.
You can check your basic against my Sysudoku basic trace.
My inner engineer asked me why I continued to put the naked pair “np” and other subset symbols in the trace. He’s right. It’s clear and consistent with the clue marks to just list the numbers in any subset, after the box name.
When I had nothing better than a coloring trial, I did look at several X-panels with slink corner formations that might add candidates to the trial sets. The 8-panel illustrates the idea. In the cluster on the corner, here’s only one pattern that includes 8r1c2 and 8r4c7. There are four green patterns, reaching every other 8-candidate. I pick blue.
Only Extreme 347 is useful as an illustration of the trial trace and corresponding graphic demonstration. In the trial trace, instead of finding all effects of each cause, we list only the first level effects, of each item on the list, returning on the next line to pick up another level of effects.
Here is the trial trace for the blue trial, arriving at a contradiction when two 2’s are forced into r4.
In a graphic overlay, we trace down from cause to cause on a direct path to the contradiction, ignoring effects that have no bearing on it.
Going back to the trial setup grid, we use clue cause-to-effect arrows and supporting weak links to display the shortest, most direct path to the contradiction.
Here blue candidates see two candidates in r9c1, starting a chain of inferences that place 2r4c4, and 2r4c8.
Now knowing that green candidates are all true, we can use normal depth first marking to reach an immediate solution.
Next, the final post of 2017 takes up the final Sudoku of the Only Extreme review, 390. Another review will follow, but in 2018,a series of pages will be added to consolidate and illustrate Systematic Sudoku developments in these seven years of the Sysudoku blog.