Dr. Sudoku Prescribes #67 – Sudoku
or solve online (using our beta test of Penpa-Edit tools)
Theme: Logical solving theme
Answer String: Enter the 5th row from left to right, followed by a comma, followed by the 6th column from top to bottom.
Time Standards (highlight to view): Grandmaster = 6:30, Master = 10:30, Expert = 21:00
Solution: PDF
Note: Follow this link for other classic Sudoku. If you are new to this puzzle type, here are our easiest Sudoku to get started on. More classic Sudoku puzzles can be found in The Art of Sudoku, The Art of Sudoku 2 and in our beginner-friendly collection Intro to GMPuzzles by Serkan Yürekli.
Do you have any idea how frustrating it is to know that all I have to do is just go find a solver program, and this would be done? Or even just start trying some things to see if they fail. But no, I have to try to stick to finding the logical solving path. Gonna die…
And I had to finally give in. Couldn’t see what I was missing, so I got help, but only so far as the one tricky step that I wasn’t visualizing. I really need to figure out how to improve my sudoku abilities; should have done better. (No idea what the actual time was since I was doing other things, but I probably banged my head against this one for over an hour of actual staring time.)
Well, my eye was drawn to what I think is the tricky part (middle squares of 4 of the 3×3 squares), but I needed copious notes and trial and error to resolve one of those squares. From there, it was steady progress to the end.
Yeah, that’s the bit that tripped me up. All I needed was to have it called to my attention, as I had the right notes marked, just didn’t realize the implication. Once I saw that there was something to be gathered from that, the rest of the puzzle fell pretty quickly.
Actually, maybe the good Doctor could address this. I decided that I need to try to work on my sudoku skills, so I went looking for some more sample problems to work. One of the ones I found was this 2010 theme from a familiar puzzle maker.
The writeup regarding Nikoli style sudoku was quite interesting, as was the discussion about trying to construct to that standard. I’m curious, though, whether this puzzle was intended to be part of that fold or not. That step with the center cells seems like it stepped outside the lines of the deduced fourth rule. Is there a simpler way to solve that which I (maybe we) missed? Or was that just the key to the puzzle, and was intended to be harder than “simple heuristics”?
(Don’t get me wrong, I’m not complaining in any case. It was definitely a fair puzzle, just tough, and my relative inexperience made it harder than it needed to be for me.)
I was planning to describe Friday and Saturday’s puzzles a bit more in the Ask Dr. Sudoku post this week. Each was set up to test whether the solvers would describe “guessing” or “bifurcation”. That this sudoku has a single necessary example of a hard rule that I consider less fun/natural but that the sudoku community views as perfectly legit — indeed one I said I strongly dislike in 2010 and still do today — is something that as desired split my test-solvers into two camps. Those who were like “I knew there was something in R28C28 and eventually got it” to those who said “what!?!”
In a competition I would probably have started after five digits with a guess, in one of the four constrained cells that actually contain this deduction. So I would, as a solver, run into this step as I target the exact kinds of spots where Y-wings sometimes occur (or other rules too). I’d say Friday is a puzzle I would do by logic, and Saturday by bifurcation.
This is actually one of those techniques I like, as I find it easily recognisable as a pattern. And in my notation style this technique is for me generally much easier found in Sudokus than x-wings.
Also, I offer these other two links to “very hard” puzzles I’ve released that might cross the 4th standard:
“Too hard for competition” sudoku
The Art of Sudoku Sampler #2
One thing that came up in the discussion in the post you link to, and that I have found looking more deeply myself, is that Nikoli has used (though rarely) steps like X-wings in their puzzles. My view is still to minimize their occurrence to only a few cases, but if they are well telegraphed, as I believe they are in my constructions, they can be fair game both as teaching tools and as actual puzzle steps. Certainly Saturday steps and not earlier!
I agree, your hidden breakthrough step was well-telegraphed. The solve still took awhile as I needed to employ many pointing pairs to eliminate possibilities. Quite a logical solve though, and a nice diversion to my otherwise hellish work day… (Of course, it didn’t help trying to solve a Sudoku on minimal sleep…)
-Allen
I’m probably not as good at picking up on telegraphing like this yet, but I did go try those two puzzles. Thanks much for posting them; it took me quite a while to find the key in the first, but the second of the two fell much more quickly. I’m going to go searching around for more examples that use these, and maybe I’ll start finding these a little more frequently.
I spotted R28C28 “relatively” quickly (relative since I am slow at sudoku), but somehow managed to got myself stuck for minutes since I missed a hidden single 9 at the lower right…… It might have to do with my habit of writing down choices for cells with exactly 2 possibilities down.
Anyways, what I want to know aside from “what constitute as logic, and what guessing” is “when to start guessing”, especially on a competition.
I meant the habit of writing down cells with 2 possibilities lead to me spotting R28C28, not missing an easy digit that I should have spotted. The latter was probably due to a StarCraft stream I have opened while solving.
I solved this puzzle with an asterisk; I failed to see the telegraphing of the choke point (what with notes all over the place before I was ready to pick up on it), and only found the choke point by reading the “Ask the Dr” post following this one. This isn’t too surprising, I suppose, given that I have not studied up on sudoku enough to know what the name of the tactic in question was.
The puzzle itself was not to blame, of course; it was up to your usual high standards, even if the solver in this case wasn’t. 🙂
3:12 on this one; I didn’t spot the trick immediately and wandered around for some time, but this still led to useful deductions so I probably didn’t lose too much time doing it. Nice construction and pleasant solve.
Beautiful Sudoku.
14 mins!
It would be nice if more of these Y-wings are employed in the solve.Actually i did not spot this as a y-wing.Both of the possiblities of 2 and 5 lead me to a 7 in Box-1 provided you have made all the neccessary pencil marks in Box-1.So i dont think my process of solving can be called as a bifurcation.After going through the comments i realised that it is a Y-wing hidden sweetly.XY,YZ,ZX,XA leads to A.Tnks to motris.I think i will keep a watchout for Y-wings more from now on.
I don’t feel too bad about struggling with this one after reading some of the comments from more competitive solvers!
Para, what kind of notation style do you use that makes these easier to spot?