PencilPress 41 and A Well Hidden Dublex

In the review of PencilPress Extreme Sudoku, 41 is a another bypass blow out, with two steep collapses. Tom Sheldon’s Master Class 50 hands you a hidden dublex that is a challenge to spot and verify. We’re reviewing the PencilPress collection for the first time, and updating the Master Class review of 2013.

Let’s say you are reading the basic trace of  Master Class 50 and get to the  NEhdx5.

Your grid copy looks like this:


Now its up to you to explain how SW5 can create a hidden dublex 5 in the NE box, and where it goes.

What’s a “hidden dublex”? Welcome to Sysudoku, and look in Sudoku Speak or The Guide.


Now while you’re trying to think,


here’s the bypass trace for PencilPress bottom right, page 41:

With the effects of C7 posted, E9 triggers the remaining collapse. This is not extreme Sudoku, but PencilPress is right in their cover page advice to older Sudoku fanatics like me: “If you play Sudoku daily, you will start to see improvement in your concentration and overall brain power.”

Now before confirming your answer to the hidden dublex problem, here is the bypass collapse of Master Class 50:


OK, now about that hidden dublex, the pencil mark 5’s lead the way from SW5 to NEhdx5.

That’s looking four steps ahead, but it’s a regular follow up in the bypass, not a search. One slink leads to another. The only trick is to see that N5 excludes 5’s from Cr5 as well as Wr5, forcing a 5 in Er5. That means the 5 in c9 has to be in NEc9.


Sysudoku posted traces abound with subtleties like this. Many of them, like this one, are revealed in box marking.

Next week, it’s PencilPress 49 (left) with the same result, but a different profile of bypass collapse. Master Class 60 (right) falls in line marking, due to an improved Sysudoku Basic. This preempts a regular XYZ-wing and three XY ANL from the earlier review. If your Basic gets there better, show me.

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PencilPress 33 and Master Class 40

Another test of your Sysudoku Basic, and another witness that the PencilPress Extreme collection is basic level. On the other hand, that doesn’t make it dull. The bypass is actually a worthy challenge, but you have to experience it to know. Here is the bypass of the PencilPress Extreme bottom right 33.

Copy the givens from last post and make the moves, explaining how the obscure ones work. The walkthrough of Sysudoku basic in the Guide includes trace reading, but you can interpret it just by taking it as saying what you would do next.  If you really can’t follow it, there’s a page that lays out in detail the plan and structure of Sysudoku traces.


Now compare your bypass grid on Tom Sheldon’s Master Class 40 with this one.  Looking back, was the 3-fill 258 in r5  available in r5 when E17 was added? The trace can tell us.

In the bypass trace below, we look at the positions of C3 and C6, relative to E17.


C3 was placed earlier than E17, but C6 comes later. E17 made r5 a 4-fill. Going down the near collapse, C6 indeed triggers the 3-fill, just before the collapse dries up. Now we can look back up through the collapse to make sure no effects can be dragged out to the right to resume it.

The trace makes other comparisons possible. For example, Master 40 gave up a huge bypass, but escaped collapse, holding on until, in late line marking, it gives up a hidden triple.

Sadly, too many expert writers bury these basic dramas, dismissing basic work as “singles”.

Next week, it’s PencilPress bottom right, page 41 at left and Master Class 50 on the right.


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PencilPress 25 and Master Class 30 Update

This post gives the third in a consistent series of bypass collapses of the pre-selected review puzzles from 500 Sudoku Extreme Puzzles by PencilPress. This is verifiable fact because readers can reproduce the sequence of grids of a solution path,  using a Sysudoku basic trace, such as this bypass trace of ppbr 25

This post also begins  an update of the first Sysudoku collection review, in February 2013, of Tom Sheldon’s Sudoku Master Class.  In the original review, only highlight grid snapshots were published. When the update review is complete, a link to it will be inserted in the original post.

On Sheldon’s Master Class 30, which didn’t get a grid display in the original review, here is the line marked grid at a point where solution is assured.


The original review’s trace didn’t separate out the bypass, so it wasn’t evident how stingy the givens were.

From now on, I’m shortening the box marking lists by leaving out the “m” (for marking) on slinks of the list value.

That modifies the line marking above to

The Sysudoku trace remains unique feature in Sudoku literature, for some reason. Everyone is welcome to use it. It’s hard to appreciate what ‘implication by indentation’ does for the basic trace, unless you try to read the form it took when I started tracing.

Below is a sample of my earlier line marking trace, in which the operator ‘=>’ means “implying” and the nested series of uncompleted lists is represented by nested parentheses. You can encounter the problem by attempting to rewrite it in the indented form I write now.

Next time, its Pencil Press bottom right, page 33 at left, and Master Class 40, to the right.


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PencilPress Bottom Right 17 and Moito III-4

The review of PencilPress 500+ Sudoku Extreme Puzzles continues with bottom right, page 17.  Along with that, we return with Sudokuwiki to Moito Road to Mastery III-4, for an alternative solution without a trial.

There is nothing extreme about PencilPress br 17. The value 4 is completely placed immediately, and the S box is filled before a plunging collapse leaves many effects in reserve, but never needed as causes. That counters any claim of being “extreme”.


Moving on to Moito III-47, the basic trace was described as routine in the original post, and a basic trace wasn’t given. You can read it by lifting the givens from this grid showing the naked triple and naked pair in line marking.

Before my party got started, Sudokuwiki uncovered two hidden unique rectangles I missed. The r45c37 rectangle is a Type 1, with two slinks from the corner opposite the bv. 2r5c3 cannot stay, because the removal of 3r5c3 both adjacent corners forces 3, making a reversible rectangle with a second solution. The r12c68 rectangle is a Type 2, slinked across two boxes. Figure out why 4r1c7 must go.

The hidden UR removals are indecisive, and were carried along in the original post.

My first finds, an XY-chain extended by a grouped 4-chain, and 9-chain ANL, boxline and naked triple it enables, were skipped by Sudokuwiki’s going directly to AIC building, with this AIC confirming ANL. Such a creature can be spotted by starting a boomerang from a bv cell.


Follow up marking


includes the naked triple at left.


Then the long XY ANL below triggers the collapse of Moito III-47, starting with:

The color trial is avoided.














Next post, it’s PencilPress Extreme 25, offering a challenging bypass.

Beginning also is an update on the collection of the review of an early “tells all”, which led me to the conclusion that tracing is a necessity.






In that first collection review, the full results for each review puzzle were not published. It soon became the Sysudoku custom to do that, so this update will follow suit for Sheldon’s Master Class. Here is the starting grid for Master Class 30.



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PencilPress Bottom Right 9 and Bean VI-24

PencilPress  500+ Sudoku Extreme Puzzles rates in Sysudoku as basic level, with most of the preselected puzzle on the bottom right corner of pages 9, 16, . . ., being solved in the bypass. In each of the posts verifying this, an advanced topic is also included. This time it’s an alternative solution, without trial, of Rebecca Bean Extremely Hard, section VI, # 24.

Any trace of a solution in the bypass is going to look complicated, but the most moves are simple. Then, as you read the trace by filling the grid, some moves are not. The value of reading a trace is that the reason for the move forms in your own head. Always formulate the reason there before you update the grid. It’s profitable to think also of why the current move was spotted. What change in the grid prompted it?  Sysudoku basic traces follow a protocol. You don’t have to know what it is, but after reading a few traces, you’ll know it anyway.

Here is a bypass trace for bottom right 9:

It starts with the short 5-chain ANL found before coloring. But that blinded me to the 2-chain ANL running alongside.








The elimination sets up two slinks for a grouped skyscraper ANL.





The coloring trial is still available, but Sudokuwiki goes for AIC building, and finds


this confirming ANL.  You or I might be on a boomerang search, but  arrive at a slink into the starting 8 instead of a wink into  2 or 5. That’s a bonus for looking.






The new clue S8 is decisive:

For next time, you can bypass your way through PencilPress bottom right, page 17,




then see if you need to read my trace.

Advanced readers get another go at III-47 from Moito’s Road to Mastery, which was finished with a coloring trial in the post of 1/02/18.





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Lucky Panda 480 Ends the Review

The last review Sudoku of the Lucky Panda Books 500 Sudoku Puzzles, 480, finishes on message with Sysudoku’s choices of bv mining tools, the XY railway and coloring. If you have time for nothing else, you might try your hand on the line marked grid, before peeking.

This puzzle is suggested as a typical example for choosing lines to mark as the number of unmarked cells increases.






The line marking trace shows that after lines of 3, 4 and 5  unmarked (free) cells are marked, four rows of 6 free cells are selected, over two columns of 6 free cells, and two of 7 free cells.

The box marking trace explains why I’m dropping the “m” (for “marks”). The “t” will remain, to mark aligned and unaccompanied triples that sweep cells in neignboring boxes. Also remaining are outside the list values and special marking symbols, such as Ens3 and SE3t above.

The first advanced move is a good example of finding ANL and nice loops along the XY railway. The railway links candidates in separate bv that see each other.




This railway turns out to house three XY terminals of value 7 for five ANL removals and two of value 6 for one ANL removal.  Then a connecting spur forms a nice loop for three more.





After the follow up

a blue/green cluster, started in the NW box, expands to trap 4r41c.












The follow up expansion of the cluster


wraps blue.



NW5 removes blue 1r2c1. The green collapse is immediate.









This wraps up the Lucky Panda review.  The review table belies the cover claims of being “insanely difficult” or “driving you to the peak of Sudoku mastery”.

On the contrary, the experience you get from these puzzles is limited to the occasional nd easy advanced.

Next is a review of another 500+ collection claiming to be “Extreme”, this one by PencilPress. It isn’t at all extreme, but the book is entertaining for those working on the bypass, so the plan is to checkpoint one review puzzle per weekly post, with some exploration of interest to advanced readers in each post.

The review starts with the bottom right hand puzzle on page 9, and continues with the same position every eighth page, that is 17, 25, . . . The reason for this offbeat pre-selection is that the 500+ puzzles aren’t numbered. Buy the book, and you’ll have time to get ahead of me, and catch violations of bypass protocol, as well as mistakes.

The advanced segment of the next post will be an alternative resolution, without trial,  of Rebecca Bean’s Extremely Hard, v.10, section VI, 24.

The multiple cluster trial solution was posted March 22, 2018. The archive menu on the right gets you there.




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Lucky Panda 432 Displays a Reversed Boxline

Lucky Panda 432 offers a clear example of a less encountered type of boxline, on the way to becoming a Sysudoku Basic casualty.

grid showing reversed boxlineIn line marking, the newly formed naked pair 13 removes 3r2c4, leaving two aligned 3’s in the North box. One of these must be N3, so the remainder of c6 can have no 3’s. Sysudoku labels this interaction of box and line a boxline.

A reversed form of boxline would occur if 3r8c6 were removed instead, leaving no 3’s in  c6 except the two in the N box. This would require the 3 of c6 to come from r12c6, removing 3r2c4.

basic traceThe details are all there in the basic trace.

Next is the conclusion of the Lucky Panda review, with 480.


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