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Innies and Outies
This is a fundamental strategy for Killer Sudoku puzzles of any difficulty. It is extremely versatile and worth looking out for it at every stage. Fortunately it only requires simple addition and spotting a certain type of pattern.

Since the basis of the puzzle is Sudoku, each row, column and box will eventually have solutions which always add up to 45. If we can identify a group of cages which almost covers a unit (or more than one unit) but has one cage either sticking out (an Outie) or poking in (an Innie) we can make some very useful deductions.

I'm going to refer to Block as any set of adjacent rows, columns and boxes we can use in concert, but also a block could be just one row, column or block. Innies and Outies occur naturally because cages often line up with the Sudoku elements but in the middle and end game you can split the cages that contain solutions and create new useful blocks that were not available at the start.

Single Cell Innies and Outies
I've deliberately split this strategy in the solver between the easier-to-spot single cell Innies and Outies and the multiple cell ones - which require more mental calculation and therefore go further down the strategy order. Lets plunge in.

You will see two examples in the first diagram. This Killer Sudoku is symmetrical - which will often yield a double Innie and Outie.

The two red rings surround boxes 3 and 7. Both boxes contain complete cages apart from one two cell cage. Lets add up the clues for complete cages in each box. We have:
Box 3 = 22 + 14 + 6 = 42
Box 7 = 6 + 16 + 18 =40

We therefore know that Box 3 is missing 3 which can only go in C7 and Box 7 is missing 5 which must go in G3. Because we are dealing with simple 2-cell cages we get the other halves as well.

If we proceed a little further down the puzzle we get another useful Innie and Outie, this time made up of two columns. Since we are using two columns, the number we are interested in arriving at is 90. On the left hand side ringed in red, the cages are 9 + 12 + 9 + 15 + 14 + 6 + 16 = 81, so 9 has to come from the 3-cell cage totalling 18. 9 can be placed in J2. As the puzzle is symmetrical a similar technique can be applied to the other side.

I'm grateful to Deelight in France for this example. This shows a group of four boxes and an Outie in C3. The total of four boxes is 180 and if we add up all the cages including the cage starting in C3 we get 181, so 1 must exist outside the four boxes. As C3 is the only outie cell that's where 1 must go. Note that we are able to consider these four boxes as a block with a single Outie cell becuase we solved the cell in G7. We can take the cell clue (12) and take off 2. That's how we get to 181.

Although the solver cannot as yet complete this Killer, you can Load it here and see the example.

The Pseudo-Cage - multi-cell Innies
There is no restriction on the size of Innies and Outies - merely what is useful, and this goes back to combinations that fit certain cage sizes and have certain clues. Some cages are plane unhelpful, for example 24 spread over 4 cells. There are eight possible combinations from 1/6/8/9 to 4/5/7/8. 24 over 3 cells is much more interesting: 7/8/9 can be the only fit. So how about looking for Innies that are two, three - maybe four cells that have very low clues values or very high clue values.

The great idea about multi-cell Innies and Outies is that they don't have to fall over the existing cages the puzzle designer has made. You are creating new cages to restrict the candidates in the same way, but are made up on the spot from rows, columns and boxes - entities you know much add up to multiples of 45. I like to think of these are virtual or pseudo-cages - fleeting but useful.

Take this Killer Sudoku and the situation in box 6. Four cages are entirely inside the box and these have clues 8, 13, 14 and 5 which add up to 40. The remaining two cells in D7 and E7 therefore must add up to 5, to make a total of 45. It doesn't matter that these two cells are not in the same cage. D7 and E7 form a pseudo-cage of size 2 and a 'clue' of 5. That could be 1/4 or 2/3. Either way, we can remove 6/7/8/9 from those cells. It gets us closer.

Here is a bolder four cell pseudo-cage using the top row as the block. We have one cages entirely inside the row (11) plus 1 and 3 (solutions in A3 and A5) gives 15. So 30 must be spread over four cells. 15+30=45. 30 is a good number since only 6/7/8/9 adds up to 30 so all the numbers below 6 can be removed. That gives us a solution of 8 in A4.

Just to show truely large 'blocks' can be useful, here is an example of four rows in combination. Looking at the design of the puzzle you can see so many cages convieniently lined up it's begging for some Innie/Outie identification. At this point the top four rows must add up to 180 and two cages stick out. The two cells inside the block must add up to 8 since the total of all those cages is 172. As the two cells are in the same row 4 and 4 is out. So to is 8 and 9 and we can remove those candidiates. This is a fun example to step through.

Using Locked Sets to Split Cages

 I was considering a puzzle idea sent to me by J Ignacio Fernandez from Spain when his idea kicked off another idea in my head. We were both looking at row G in the next puzzle. I got the notion that we could used Locked Sets to expand the number of Innies and Outies eliminations. In the puzzle sent to me we have a long 5-cell cage valued at 33. The bottom two cells G3 and G4 contains a Naked Pair - a simple locked set. Because we know 4 and 8 must go in those cells (just not which way round yet) we can sum those values and let them contribute to the block we are making. So adding 7 and 7 (first and last cages in row G) to the 12 (4+8) from G3 and G4 we get 26. So we need 19 to fit in the last three cells G567. Combinations of 19 in three cells means we can remove a candidate in each as shown on the diagram.If the solver makes use of Locked Sets it will report this info alert:(*note: the cage that includes C3 has been split to include it in the block) Locked Set used to make Block

 Go back to Cage/Unit Overlap Continue to Cage Splitting

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## Thursday 16-Jan-2014

### ... by: Jigancio

Hi

I'm sorry, but alas! this 'only' is not a solution, it gets stuck again a little later "run out of valid strategies"

Yes, of course, as I posted, g5+g6+g7=19, and the same g5+g6 plus f6 sum up also 19 (box).
Thus, necesarily g7=f6, that's the trick, and we can remove 4 from f6 and 9 from g7, what now leads to the solution (j7=9 and so on).

No doubt, you have improved the solver, but in my opinion it still lacks a routinary and effective way of detecting which squares are the same value in order to inmediately eliminate single candidates.

Sincerely yours

J. Ignacio Fernández

Andrew Stuart writes:

I understood your trick but dont have a way of generalising it yet. I agree my enhancement doesn’t solve this puzzle but it's very useful over many puzzles. Still working on your strategy. I'll post more when I get there!
Thanks

## Sunday 7-Apr-2013

### ... by: PeteTy

todays killer took me 40min was very challenging

the thing that broke it for me was the c8-c9 triple innie 11

http://www.sudokuwiki.org/killersudoku.htm?bd=122111122113322212122113311332122212313313332213212113212212443231331211331131233,250900260000001300000007001000002000000600200009000000190000001700000015001518001209000000150000210008040014000000000000080000000920130011150700000000000000001000

so i loaded it in the solver
r78 single innie
n9 single innie

row 1-4 has dual outie 16 for a 7 9 pair

the solver spotted all those

box1 has a dual outie of 2
i noticed that before the singles but,
the solver never did spot it .. solver found the 1 to the south with some magic multiple cage interactions and exotic hard cage combinations

after it found the 1 to the east in the 7(2) cage,
it took several solve steps before it discovered the 6
I dont consider that particular combination to be a tough one ...

did notice the solver has:
Innies (2 to 4 cells)

doesnt have:
outies (2 to 4 cells)

a little thought will confirm outies dont have the cage convention (no doubles) while innies do
---------------------------------------------------

order of the steps the way i solve vs the solver

ill do cage splitting innies outies cage combinations way before hidden triples quads

way before y wings, colouring or pointing pairs

i really think the tough strategy on the killers shouldnt start before the normal sudoku tough strategy

perhaps move the blue ones (killer specific) to lower numbers

## Thursday 8-Nov-2012

### ... by: PeteTy

there are a few caveats involved with multiple innies and outies
just look at the lack of the killer cage convention

a fun example (last above) gives N1+N2
an innie 36 or 90-(22+15+17)

a 9 cell pseudo-cage
it cant have 6 or 9 because they are required in the 22(3) and 17(3) cage

possibles
Digit Combinations - Can't Have Digits 69 Must Have Digits1234578
9 cell cage
36: 112345578* 122344578 123334578
Digits 1234578 Combinations 3

just look at the lack of the killer cage convention

n2 above sums to 14(4) outie
n8 has a 32(5) outie
both of these happen to have no repeated digits

r5 has an outie 14(3) very easy to spot
r1+r2+r3+r4+r5 has an outie 6(1) a single
which leaves an outie 8(2) for r5 to r9
its confined to row 4 and must have no repeated digits so 44 is not in the possibles

when i start getting stumped with a killer i may see n1 with a 2 cell outie
one goes east the other south
outie 4
2 cell cage
4: 13 22
Digits 123 Combinations 2

outie 10
Digit Combinations -
2 cell cage
10: 19 28 37 46 55
Digits 123456789 Combinations 5

outie 18
2 cell cage
18: 99
Digits 9

## Saturday 3-Nov-2012

### ... by: DavidC

Thanks for a great site. In the pseudo-cage section of Innies and Outies, you say "D7 and E7 form a pseudo-cage of size 2 and a 'clue' of 5. That could be 1/4 or 2/5." There is a typo : it could be 1/4 or 2/3. Regards, David.

## Thursday 6-Sep-2012

### ... by: MK

@akansha

So the concept of Innies and Outies is this: since any given row, column, or box of nine squares contains the digits 1-9, it must add up to 45 (1+2+3+...+9=45). Therefore, if you have a given set of cells that overlap said row/column/box, plus an extra square (an outie) or minus a square (an innie), you can calculate the value of that square.

Example: a row made up of the followings four cells (the numbers are the sums, underscores is a blank square:
[12 _] [12_] [12_][7_] [_]
[_ _ _][ _ _ _][_ _][_]
The last underscore is part of a two-square cell overlapping the row below. In this case, it's an Innie, because all the other cells are contained in this one row. So you add the sums of the cells: 12+12+12+7=43. You know that the total sum of the row must equal 45. So, 45-43=2 means that the square on the end by itself must be a 2.

It would work the same way for an Outie, except that the total sum of the cells would be greater than 45 and the empty square would be on a different row. (Example: total sum is 48. 48-45=3, so the 3 goes in the Outie square.)

Then you just expand this for multiple rows/columns/boxes. If the sum of one row is 45, then the sum of two rows is 90, the sum of three rows is 135, and so on.

I hope that this helps to clarify. I know that it's hard to visualize when I don't have proper pictures like above.

## Monday 4-Jun-2012

### ... by: akansha

Could not understand the concept of Innies and outies.
Can you be more elaborated/descriptive?
With example.

## Monday 17-Oct-2011

### ... by: Kaylea

I didn't know where to find this info then kboaom it was here.