Castillo’s Only Hard Sudoku


This post concludes that Manual Castillo’s Only Hard Sudoku book is for those sysudokies who enjoy the bypass challenge, and are not expecting a to get to many advanced techniques.

So many puzzle books, so little time. This relatively recent 2014 release by Manuel Castillo, lives up to it’s name, in that it does not venture beyond a simple “hard”. It is varied and active in basic operations, and sometimes challenging, but the collapse usually comes before all candidates are corralled for advanced solving. I preselected one every 40, starting with Only Hard 2. Here is the standard Sysudoku review table.

Only review table

Plenty of Wayne Gould “no pencil marks” runs, and plenty of hidden singles and naked pairs and triples. Any “Only” that makes it into advanced solving will come with plenty of bv cells, and I would not expect any to survive the bv scan.

Here is the checkpoint line marked grid for the one taking up the most space in the review table, Only Hard 82, with fill strings still attached.

Only 82 LM grid

To get to this rather spectacular line marking, my bypass was short but tricky, requiring some of that backup from box marking that is convincing me to do box marking on the same sheet. Said line marking was a series of naked singles, each playing out where I had come to expect a collapse.

Only 82 basic tr

Note how thoroughly and compactly the complex, systematic shredding of Only Hard 82 is documented by the sysudokie trace.   The 8-wing on the line marked grid appeared in the marking of line r5 when the naked single E6 led to the NE1 clue, removing 8r1c1. The wing icons stayed on the grid through the remainder of line marking, and kept the 8 off the fill string for c6.

The close procedure yielded a naked quintuple, and on the last line, a naked triple. And did you notice how the row 8 naked triple removes a candidate in row 7? This had no chance to impact, however, as the quint marks to a collapse.

only 82 type 4 URToo bad, but the naked quint erased this Type 4 UR. My approach is to spot it first by the extra candidates 4 and 9, then look around for somebody to get rid of them both. Of course that somebody, 4r1c4 in this case, becomes the victim of the UR. Or you can use the Type 4 drill procedure, taking extra candidates 4 and 9 as contents of a cell on r1, completing a mental naked triple that forces out the offending 4.

Again, Manuel’s Only Hard Sudoku collection is an active basic marking challenge, with an occasional surprise.

Next time, another Unsolvable 190, which offers me and JC an exocet which, with some effort, I refuse.

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About Sudent

My real name is John Welch. I'm a happily married, retired professor (computer engineering), timeshare traveling, marathon running father of 3 wonderful daughters and granddad to 7 fabulous grandchildren. The blog is about Sudoku solving. It covers how to start, basic solving to find candidates efficiently, and advanced solving methods in an efficient order of battle. It is about human solving methods, not computer solving.
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