An Error Hides a Lite Coloring Trap on ultrahardcore 399

This post corrects an error on the post of December 8, that prevents a lite coloring trap in ultrahardcore 399. Then there’s follow up on the trap, with the lite coloring marking in place. Trial is not avoided, but the lite coloring hastens the final wrap by contradiction.

Here’s the correction on a  goof by yours truly. On the APE by Sudokuwiki  and ALS_87 by Beeby, I deleted 7r3c8, instead of 7r2c8. In the post, that error is carried forward without incident, not affecting any eliminations or the coloring trial which solves 399.

Now coloring lite has a trap, on that very 7r3c8 spared by my error.

7r3c1 is blue lite, 7r6c8 is green lite.

The removal does little damage to UHC 399. Realizing that, I enlisted Beeby by stepping through its former finds, and using its REMOVE feature, to take out  the lite trap’s 7r3c8.

Beeby came through with one more removal,  Take it as a boomer from 7r7c8 or a 1-way from 3r4c8, depending on your starting point.

Beeby runs out of human options now, and I put in some time on winning one for lite coloring, but  couldn’t. Maybe you did.

There are alternatives to the green coloring trial of the review.

Cc4 contains 6(1+8)(4+5), so orange comes with a C45 pair. The SASdC in Nr1 requires a trial of Nr1=572.

Also there’s a pattern situation on the 7-panel left by the lite removal. You could test the three patterns containing 7r7c9, 7r6c8, 7r8c3 by including them as clues and dropping the 6 orphans. If that trial fails, you would know that 7r2c9 and 7r3c1 are true, and that 7r2c5 and 7r2c7 are orphans.

However, since we get a boost on the green trial as 7 green lite candidates turn green, we’ll take that. Comparing to patterns, if the trial fails, we get 6 clues and 6 removals.

The point is that lite coloring also enhances trials.

This time, compared to the green trial in the review, the green clues change little, because we have already incorporated most of their effects. The color expansion is in the red/orange cluster, which grows in a distinctive way, by adding naked pairs.

looks like I may have to try the red or orange army. Then I notice that in r2, orange will enable a now naked triple to confirm 4r2c5 and then red 1r2c1. No, Virginia, orange doesn’t get to confirm red. It gets tossed for trying, and collapse follows.

Compared to the review post, the color solution has the same reds, but a few more greens.

There’s now a lite coloring page in The Guide, mostly based on last week’s post. It’s a child of the coloring page (drag off right). Guide pages on Sysudoku trials are coming soon.

As to posts, the next one will look back to a previous attempt to extend coloring. Suggestions on collections to review or topics to explore are welcome, either in comments on posts or at Please attach comments to relevant posts.

About Sudent

I'm John Welch, a retired engineering professor, father of 3 wonderful daughters and granddad to 7 fabulous grandchildren. Sudoku analysis and illustration is a great hobby and a healthy mental challenge.
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