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[Petrus] How to Fix Bad Edges (Video & Text)

JLarsen

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[youtubehd]mFvG7bb18II[/youtubehd]

Introduction:
So, starting off, let's define what a bad edge is, and why we fix them;

A bad edge is one that cannot be solved using only 2 sides, which in Petrus are typically R and U. Let's give an example; scramble your cube using only 2 sides, and then hold the 2x2x3 block on the back. Perform these moves; R U R2 F2 R. The bad edges here are at RU, FD, FR, and FL. Just accept it for now - I'll explain how I identified these as bad shortly. Now try to solve them. No matter what you do, you cannot solve those edges without breaking the 2x2x3 block. The goal here is to make it so that we can finish the F2L of the cube using only 2 sides, so we must flip these bad edges.

How to identify a bad edge

Beginner- Moving the edge between it's two centers does not solve it.

Advanced-

1. F color sticker is on U
2. U color sticker is on F
3. D color sticker is on F
4. Any sticker facing away from its center.

How to fix them

Redo that sequence I first mentioned if you've done any moves to mess it up. (R U R2 F2 R) Now look at the RU edge. We can see it is bad, because performing U F2 moves it between it's centers, but does not solve it. Undo those moves if you did them. Without doing this, we can tell it is bad because the F color, is on U. Now, do an R' while watching the edge. See how that made the edge good? But simply undoing R' with R makes the edge bad again. Notice that when we bring a bad edge into the other layer using R or L turns, the edge becomes good.

Now, apply R' again, which makes the edge good, but this time, replace the edge on the F layer with another bad edge before undoing the move. We can see that the DF edge is also bad, so if we do R', to fix the RU edge, then perform F' to move another bad edge in it's place, and then do R to undo our R' again, we can see that we have fixed 2 bad edges. You still with me?

Now, our last 2 edges are at DF and FR. Note that to fix bad edges, the 2 edges you are fixing must be in different slices. If we do F', the FR edge is now in the U slice, and the DF edge is now in the R slice. We can now make the FR edge good by performing R, replacing the now good edge with a bad one in the same layer by performing U', and then undoing our original move with R', to complete our bad edges. If you now hold the 2x2x3 block in you left hand, with the two free layers on R and U, you can solve the remaining f2l with only those 2 sides. This is the purpose of bad edges. Go ahead, give it a try! Success is sweet.

More Examples

*Scramble with 2 sides before performing these and hold the 2x2x3 on block for all of them.*

Scramble:R U' R' L F L'

Our bad edges are at DF because F is on U, UL because the U sticker is away from it's center, FU because F is away from its center, and BU because U is away from its center.

L to move UL into the opposite slice, making it good.
F to replace the edge with the edge at DF.
L' fixes the other edge we are flipping

Our last two edges are at FR and UB

R moves a bad edge into the other slice, U replaces, R' flips the other.

(R U R')


What about these "special" cases I see on lar5.com/cube Mr. Panda?

You mean those 4 flips and 6 flips? They're really a lot easier than you think. Besides one special case, which I'll go over in the next example, all 4 flips are simply cancellations between 2 2 flips.

Scramble: R U2 R' F L' U' L

Bad edges are at

FU FL DF and UL

First we'll fix the FL and FU edges with

L' U L

this leaves us with DF and UL, which can be fixed with

L
F L'

Take another look at those solutions; (L' U L) (L F L')

Notice we have a cancellation between bolded moves. This can be performed as L' U L2 F L'. You have just flipped 4 edges at once. I'm not sure why Lars chose to just list a bunch of cases with arbitrary solutions. The only 4 flip that really is a special case is this, and it's still pretty intuitive;

Scramble: M' U M

R' L F R L'

All we're really doing here is bringing down a bad edge in both the Right and Left slice, and replacing both of them with 2 edges on the front.

It would work just the same if it were R L U R' L'. See how that uses the same concept?


Now for the 6 flips. I'll use one of Lars' "algorithms" to set this one up.

Scramble: R' L F R L2 U' L

R' L F R L', L' U' L

Notice how the beginning is just that fancy Roux M slice 4 flip case executed as LR turns with a cancellation into a 2 flip at the end. Every single one of the 6 flips you'll find at lar5 start with that 4 flip, and just cancel into different 2 flips, depending on the location of the good edge at the start.

So should I use 4 flips and 6 flips?

Personally, I've never bothered with 6 flips, and I never go out of my way to do a 4 flip. If you have decent lookahead, you'll be able to recognize a 4 flip pretty easily if there is one, and execute it as such. Otherwise, I just keep on 2 flipping. I'm certainly not the fastest Petrus solver out there, so don't take my word as gospel, but know that sub 20 is perfectly achievable using nothing but 2 flips. Hopefully, after reading this guide, all of the cases on Lars site make sense to you on an intuitive level, and 4 flips shouldn't take all that much effort to incorporate into your solves. As for 6 flips, maybe I'll have to give it a go myself. I'm not sure if recognition makes it worth it or not, but I've never tried. I'll update this guide if I ever get the drive to learn them. You do have to memorize which case to use depending on the position of the good edge. If I come up with a better system, I'll be sure to share.


Hopefully you all found this helpful. If you have any questions feel free to post. Good luck and happy cubing - Panda
 
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Tim Major

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#5
Even though they are pointing out errors, don't worry. Good tutorial. As you average 13 or something (not sure, sorry...) do you correct bad egdes while making the 2x2x3? I read about this somewhere after I learnt petrus.
 

JLarsen

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Even though they are pointing out errors, don't worry. Good tutorial. As you average 13 or something (not sure, sorry...) do you correct bad egdes while making the 2x2x3? I read about this somewhere after I learnt petrus.
Personally I never do that in actual solves because I don't make 2x2x3 expansions like that. Then only way this is possible is in the case where you make a 1x2 block next to a 2x2, that is a turn away. For example make a 2x2x3, and hold it as if you were about to orient edges, and then perform R'. the 2x2x3 expansion is an R turn away. If you look at the edge at FR, you can see that it will be moved into the opposite layer once that R turn is performed, and if that edge is good, you can replace it with a bad one before completing the 2x2x3 block =D. Did I asplayn dats gewd?

Thanks for the compliments, as for my times I average 15-16, with a PR avg of 14.78 =D.
 

JLarsen

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Just an update on this guide - I made some changes to wording, added some examples to cover 4 flips and 6 flips.

I've got a video tutorial on the way and I was looking for some guidance. I wanted to keep it brief but I find myself wondering at what point a brief tutorial just becomes rushed and way too fast. How long do you think the video should be? How many examples should I give? Did you like the format of this guide where I give scrambles and have the audience follow along? Should I do a lot of editing? I tried to make this guide sort of conversational instead of overly formal sounding. Did you like that?

I'm trying to make the end all be all bad edges tutorial so I want it to be really polished. That way later on down the road when I'm like 25 and looking back on my videos I don't hastily delete it out of embarrassment. I have a habit of doing that. Input appreciated!
 
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#8
My "beginner advice" for EO is - you need 3 moves to orient 2 edges, first move breaks the 2x2x3 last move restores it - this was al I remembered after reading Lars Petrus Tutorial once, but it was enough to start.


My "advanced advice" is: "EO rules apply to opposite colors, too" (Example: if yellow is top color and white buttom color. and you see a white sticker during EO treat it as if it were yellow / same appies to one B-colored sticker - treat it like f-colored)

This is a very helpful knowledge, because together with the fact that misoriented edges occur in even numbers it saves a lot of looking at the sides of the cube during EO.


Most EO moves are intuitive even the cases were you can orient 4 edges with just 5 moves. But for the 6 bad edges case I use a fixed sequence: M'UM L'U'L / M'UM RUR' / MFM' LFL' / ... (fourth move has always the same direction than first move)
 

JLarsen

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My "beginner advice" for EO is - you need 3 moves to orient 2 edges, first move breaks the 2x2x3 last move restores it - this was al I remembered after reading Lars Petrus Tutorial once, but it was enough to start.
I like this a lot. I'll definitely include it.

oll+phase+sync said:
My "advanced advice" is: "EO rules apply to opposite colors, too" (Example: if yellow is top color and white buttom color. and you see a white sticker during EO treat it as if it were yellow / same appies to one B-colored sticker - treat it like f-colored)
During some of my drafts I combined rules 2 and 3 to read 2. "Any U or D sticker on F is bad". I'm not sure if it'd be easier to grasp just mentioned the U because the D sticker only pertains the the DF edge during bad edges.

Thanks for the feedback I should be recording the whole thing on my next free day which is Thurs.
 

JLarsen

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Well done, sir.

If you're still in the mood to shoot videos, I would be highly interested in anything you have to say about 2x2x2 and 2x2x3 block-building.
Hmmmmm. Someone else requested an example solve video? I found it incredibly difficult to keep myself from rambling in this tutorial and it'd probably be even harder if I were to do a blockbuilding video. Perhaps I could just do an example solve video with some ramblings/thoughts included?
 

teller

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Hmmmmm. Someone else requested an example solve video? I found it incredibly difficult to keep myself from rambling in this tutorial and it'd probably be even harder if I were to do a blockbuilding video. Perhaps I could just do an example solve video with some ramblings/thoughts included?
I'm more interested in what/how you pre-plan 2x2x2 more than anything else. It's a fetish of mine. What sorts of tricks you use, what patterns you look for, etc. Rambling is fine...I'm not looking for perfection here, just wanting some ideas from a competent Petrus user (that's you! :)).
 
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Hmmmmm. Someone else requested an example solve video? I found it incredibly difficult to keep myself from rambling in this tutorial and it'd probably be even harder if I were to do a blockbuilding video. Perhaps I could just do an example solve video with some ramblings/thoughts included?
Just record the solve, then do a voice over.
 
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