Single Alternate Sue Unlocks ultrahardcore 223

This post reports how both review solvers run out of humanly practical options, and the arduous SASdC trial that unties the knot.

At the halfway point of the review UHC 223 starts tough, conceding four clues in basic.

Those 6f: to 8f: lines and their correspondingly long fill strings map onto a severely crowded grid.



Sudokuwiki leads off with two of its digit forcing chains, first a group mittened AIC ANL,





then a digit forcing chain boomer from 4r4c1. Then, being restricted from using cell or unit forcing chains, Sudokuwiki runs out of options.




Similarly, Beeby dries Similarly, Beeby dries up after this unlikely ALS_68.






There is a ray of hope in a nearby Single Allternate Sue de Coq, namely

NEc2 = 4(2+3)(6+9) +649.

The bv23 match in both remainders brings a clue and 8 candidate remainders. Having slogged through the review this far,  you know what comes next.

It’s a trial of the second term 649, which adopts blue and gathers enough clues to get the solvers working.

The trial continues with this grouped ANL, and a band of helpers.







First an XY ANL (black) removes 1r9c2, creating the naked pair SE25.

Then the ANL is extended to make the terminals 8 (red), removing 8r7c3.

Then the ANL is extended to make the terminals 8 (red), removing 8r7c3.

And of course,  a BARN.


The beat goes on with another XY ANL,


and after a brief follow up,










yet another one.



Trials don’t normally require a series of  AIC eliminations, but the ultrahardcore are not normal.


It takes a bit  more follow up to reach a contradiction, a second 2 in the NW box.

Since NEr2 =  649  fails,

NEr2 = 4(2+3)(6+9) . The (6+9)  alternate comes from r2c2; the (2+3)alternate,  from r2c4 and r1c8.

Next week, we go on without these 7 candidates to a solution to 223. If you already have your trial free solution, then after sending in your guidepost description to have it published here, you may want to get started with ultrahardcore  267.


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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