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 15x15 18th September 2021 
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Posted on: Tue Sep 21, 2021 4:36 pm




Posts: 817
Location: Dublin, Ireland
Joined: Tue Mar 01, 2016 10:03 pm
Post 15x15 18th September 2021
It has taken me four days to complete this puzzle! [confused] [scared]

I kept getting close to completing it, only to find an error in the last few cells. It’s telling me I spent just under an hour on it, but I figure it’s probably close to ten times that amount (or possibly even longer) if I take all the failed attempts into account.

Not sure why I found it so difficult, but I note that it has only been solved 31 times so far compared to 69 and 81 times for the previous two 15x15 puzzles.


Last edited by paulv66 on Tue Sep 21, 2021 9:05 pm, edited 1 time in total.



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Posted on: Tue Sep 21, 2021 4:49 pm




Posts: 2953
Joined: Thu May 12, 2011 11:58 pm
Post Re: 15x15 18th September 2021
For reference, and just so non-subscribers can enjoy this puzzle [scared]

Image


Last edited by pnm on Tue Sep 21, 2021 9:17 pm, edited 1 time in total.



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Posted on: Tue Sep 21, 2021 4:54 pm




Posts: 257
Joined: Thu May 12, 2011 11:51 pm
Post Re: 15x15 18th August 2021
I think you mean September 18.. The 224.8 rating may be indicative. I have not solved it yet.


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Posted on: Tue Sep 21, 2021 9:05 pm




Posts: 817
Location: Dublin, Ireland
Joined: Tue Mar 01, 2016 10:03 pm
Post Re: 15x15 18th August 2021
sjs34 wrote:
I think you mean September 18.. The 224.8 rating may be indicative. I have not solved it yet.


Absolutely. Living in the past! I’ll try to amend my original post.


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Posted on: Tue Sep 21, 2021 9:26 pm




Posts: 817
Location: Dublin, Ireland
Joined: Tue Mar 01, 2016 10:03 pm
Post Re: 15x15 18th September 2021
Spoiler alert! One of the main reasons I kept getting stuck over and over again was because I developed a blind spot in relation to a right angled 3 cage with a multiplication operator.

The value of the cage expressed in prime numbers was a^2 * b^2 * c (c > 10). I tried solving the puzzle based on the assumption that the possible cell values were (a^2 * b, b, c) or (a^2, b^2, c), totally forgetting that (ab, c, ab) was a third possibility. When I realised my mistake, I was able to finally solve the puzzle.


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Posted on: Thu Sep 23, 2021 12:29 am




Posts: 590
Joined: Wed Apr 16, 2014 9:20 pm
Post Re: 15x15 18th September 2021
I thought the puzzle was harder than most 15 x 15s. Your post scared me as I hadn't been able to look at it.

I skipped your spoiler post.

I would estimate that I spent twice as much time on this puzzle compared to other 15 x 15s.


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Posted on: Thu Sep 23, 2021 1:43 am




Posts: 148
Joined: Sun Jan 31, 2016 7:52 am
Post Re: 15x15 18th September 2021
12x12 & 15x15 : How to solve

Recently I have been taking more of an interest in the larger puzzles, and I think the 15x15 from 18th September was a little harder than usual, but still not too difficult.
I think the difficulty rating for the large puzzles is much higher than it should be, usually they can be solved without doing anything complicated. My suggestion would be that if you find yourself doing anything complicated or doing any deep analysis then stop and make sure you have done the basics first.

My method is:
Start with any cages that only have 1 combination, as you enter the pencil marks for each of these cages there are a couple of things to be aware of; if it is a 2 cell cage it will give you a pair that will be very helpful as you progress, look for matching pairs using the same rows or columns that will give a cross pair (eg cages a1 & o1 with the value 14), and if a cage must contain a value then make a note of that for use as you calculate other cages.

Next, repeat the first step, now that you have found several pairs you will find more cages with only 1 combination (eg cage a14=3,6, this now means e14(12x) has only 1 combination if we dismiss 3x4 and 2x6 because of the pair in a14)

Obviously as cells are solved remove pencil marks and re-calculate cages as required

Repeat the above steps until no more changes can be made, then do the same steps above with cages with 2 combinations, and if needed 3 combinations. I would not go any higher than 3 combinations if possible.

By this stage you should have solved several cells and the puzzle should not be too messy with excessive pencil marks.

Look for rows or columns that have the most solved cells and paired cells in them, now do a quick check of each value in those rows and columns looking for a value that is only in 1 cell, or a value that is only in 1 cage (eg in row 1 the only possible cells for 9 are in cage f1, so f1=6,9 and we go back to repeating the above steps). The puzzle is too large to look for every value from 1 to 15 in every row and every column, so look for those rows and columns where a lot is already known.

Just keep doing these steps and the puzzle will be solved, no complicated steps, just keep doing the basics.

Don’t get too worried about searching for cross pairs, they are not usually needed, but in the early stages as you find basic pairs they are easier to see, so try to notice any that become available. As you get more pencil marks they become a bit harder to notice so don’t worry about it.

I know all this might seem obvious, but if you are finding the large puzzles difficult then try it and you will be surprised how easy they can be.

Thing to avoid:
Too many pencil marks, the larger puzzles can become a complete mess if you have lots of pencil marks.
Trial and error, this will work but unless you get lucky it will waste a lot of time.
Adding cages, this might help sometimes, but not very often, I never add cages on the large puzzles.


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Posted on: Thu Sep 23, 2021 2:18 am




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Location: Toronto, Canada
Joined: Mon Nov 04, 2019 5:08 am
Post Re: 15x15 18th September 2021
Spoiler:

Yeah, I've been using trial and error, maybe as a crutch. Mid puzzle solve, quite a bit filled in but still had lots of options open.

2nd last row from the bottom had some nice options to try for trial and error, to narrow it down. But that wasn't enough to solve the puzzle for me.

Maybe eliminating an option for the 450x.

I think one reason I needed all the trial and error was because of the last row on the bottom. The 2- and 1- is actually quite the constraining option for which combinations can be used. But, it's impossible to use pencil marks, since 2- and 1- have a ton of options. So trial and error to find the conflicts.


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Posted on: Thu Sep 23, 2021 2:57 am




Posts: 148
Joined: Sun Jan 31, 2016 7:52 am
Post Re: 15x15 18th September 2021
chris42 wrote:
I think one reason I needed all the trial and error was because of the last row on the bottom. The 2- and 1- is actually quite the constraining option for which combinations can be used. But, it's impossible to use pencil marks, since 2- and 1- have a ton of options. So trial and error to find the conflicts.


I don't do anything with ages like the 2- and 1- in the bottom row until I have solved a lot of cells. From memory I think I did not give any thought at all to the 2- cage at e15 until there was only about 4 cells left unsolved in the entire puzzle and then it was just filling in the last remaining values.
There is probably a relationship between the 2- cage and other cages in row 15, but don't worry about it, do the easy things first.


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Posted on: Tue Oct 12, 2021 10:43 pm




Posts: 257
Joined: Thu May 12, 2011 11:51 pm
Post Re: 15x15 18th September 2021
Whew! I thought I was going to have to cede the all time extra points leadership to mparisi, clm, bram and manni because the 9/18 15x15 was giving me a seriously miserable time. Finally got through it.


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