# Number of ways to fold a 2x4 map

M.Gardner in his book "Mathematical puzzles and diversions" states that the following 2x4 map

$$\begin{array}{|c|c|} \hline 1 & \phantom{1} & \phantom{1} & \phantom{1} \\ \hline \phantom{1} & \phantom{1} & \phantom{1} & \phantom{1} \\ \hline \end{array}$$

can be folded in 40 different ways (along 7 segments shown) that the cell with 1 on it will be the upper one.

How to count all these ways and prove that there are exactly 40 of them?

• It's only 40. Just count them. Be systematic about the order. Horizontal and vertical folds are embarrassingly separable, and there's a tiny number of vertical folding regimes that don't leave cell 1 exposed. – greg m Jun 21 '14 at 16:28
• @gregm, that is not so easy. Try it. – klm123 Jun 21 '14 at 21:07
• @kaine, 1 is on one side. The result should be 1x1 "map". Would you mind to show your 20 ways in an answer? I will try to find as many as I can in nearest future too. – klm123 Jun 23 '14 at 13:04
• @kaine, do you take into account folds like $$\begin{array}{|c|c|} \hline 1 & 8 & 2 & 6 \\ \hline 4 & 7 & 3 & 5 \\ \hline \end{array}$$? here numbers are positions of the cells in the folded "map". – klm123 Jun 23 '14 at 13:59
• @kaine, i think you are wrong that vertical folding order doesn't matter, it matters if you put horizontal in-between. – klm123 Jun 23 '14 at 14:05

For short hand: the horizontal line is $H$, the center vertical is $C$, the one to the left of $C$ is $L$, and the one to the right of $C$ is $R$. The cells are:

\begin{array}{|c|c|} \hline 1 & 2 & 3 & 4 \\ \hline 8 & 7 & 6 & 5 \\ \hline \end{array}

## Fold $H$ last: 4 ways

$H$ and $L$ have 1 way to fold, so here can be ignored. As $C$ and $R$ have 2 ways to fold and their order doesn't matter (make dulplicate orientations) there are only $2\times2=4$ different ways to fold the sheet ignoring $H$ for the end.

## Fold $H$ first: 7 ways

If one were to fold $H$ first, there are 6 different ways to fold the sheet. 4 are simply folding $R$ and $C$ in their 2 possible ways. The other 2 involve tucking cells $3-6$ between $1$ and $8$.

The seventh was well decribed by klm123 and aschepler: Fold $Ho$ . Slide $4-5$ between $1,8$. Making it more like a cylinder than a triangle, continue sliding in until $4-5$ are between $2,7$ and $3,6$ are between $1,8$. Then you can fully flatten the LoCoRo creases, making it a 1x1 square.

## Fold $R$ then $H$ first: 6 ways

If one folds $R$ then $H$ one gets 6 orientations in exactly the same way as the first 6 option from folding $H$ first.

## Fold $R$ and $C$ then $H$ first: + 4 ways

Neither $H$ last nor $L$ last scenereos can cover square $1$ as long as $H$ and $L$ are both folded out and no tucking possible as the first two folds quickly shorten the paper. This means that there are 8 possibilitys: $RoCoHoLo,RiCoHoLo,RoCiHoLo,RiCiHoLo$ and the same ones with $LoHo$ (but these 4 are duplicates of $H$ last).

## Fold $C$ then $H$ first: 3 ways

$C$ can now only be folded "out" in the same direction as $H$ or cell $1$ is covered.
Once this is done, however, $R$ can be folded into three different orientations. The options are to place $4$ and $5$ between $1$ and $8$,$2$ and $3$ or $7$ and $6$. Folding $L$ makes it a 1x1 but doesn't change the number of patterns.

## Fold $C$ and $L$ then $H$ first: 3 ways

$C$ can now only be folded "in" and $L$ must be folded "out".
Similar to the previous case, there are 3 different options based on folding $R$ after $H$. They are to place to place $4$ and $5$ at the bottom, between $6$ and $7$ or between $2$ and $3$.

## Fold $L$ then $H$ first: 7 ways

$L$ can now only be folded "out" and $H$ must be folded "out".
This yields 7 different possibilities based on where $3-6$ are placed. There is: one with $4-5$ between $2$ and $7$; 2 ($Ri$ and $Ro$) with $3-6$ between $1$ and $2$; 2 with $3-6$ between $7$ and $8$; and 2 with $3-6$ on the bottom.

## Fold $L$ and $R$ then $H$ then $C$: 6 ways

$L$ can now only be folded "out".
This yields 6 different possibilities based on where $3-6$ are placed. There are: 2 ($Ri$ and $Ro$) with $3-6$ between $1$ and $2$; 2 with $3-6$ between $7$ and $8$; and 2 with $3-6$ on the bottom.

• Case "Fold R then H first." I see only 4 here. How do you do it in the same way as Hfirst? – klm123 Jun 23 '14 at 16:16
• (R out, H out, C out, L out) (R out, H out, C in, L out) (R in, H out, C out, L out) (R in, H out, C in, L out) (R out, H out, C out, L out, tuck 3-6 betweeen 1 and 8) (R in, H out, C out, L out, tuck 3-6 betweeen 1 and 8) – kaine Jun 23 '14 at 16:44
• Thanks. I get it. But the other 2 chapters is even harder to understand... – klm123 Jun 23 '14 at 22:09
• I think "\\$" is no longer used. You must use$ only. – klm123 Jun 24 '14 at 14:34
• Aha, yes, that's the last one. Another way to describe klm's number 5 using kaine's notation: Fold $Ho$. Slide $4,5$ between $1,8$. Making it more like a cylinder than a triangle, continue sliding in until $4,5$ are between $2,7$ and $3,6$ are between $1,8$. Then you can fully flatten the $LoCoRo$ creases, making it a 1x1 square. – aschepler Jun 30 '14 at 12:31

This is not the answer, but just one step forward it.
I give here 29 ways to fold the map + idea for 4 more ways.

I number the cells according to positions they would take at the resulting 1x1 folder map, counting from top to bottom.

Notations: H - fold along horizontal line; VI - fold along I-th vertical line; in - fold in a way that front part (usually with numbers) will be hidden inside; out - fold in a way that front part (usually with numbers) will be outside; "IJ" - cell in I-th row and J-th column.

Left-to-right order of vertical folds.

6 when fold horizontal first:

1. Hout, V1out, V2in, V3out $$\begin{array}{|c|c|} \hline 1 & 4 & 5 & 7 \\ \hline 2 & 3 & 6 & 8 \\ \hline \end{array}$$
2. Hout, Vout, Vin, Vin $$\begin{array}{|c|c|} \hline 1 & 4 & 7 & 6 \\ \hline 2 & 3 & 8 & 5 \\ \hline \end{array}$$
3. Hout, V1out, V2out, V3in $$\begin{array}{|c|c|} \hline 1 & 8 & 5 & 4 \\ \hline 2 & 7 & 6 & 3 \\ \hline \end{array}$$
4. Hout, V1out, V2out, V3out $$\begin{array}{|c|c|} \hline 1 & 8 & 3 & 6 \\ \hline 2 & 7 & 4 & 5 \\ \hline \end{array}$$
5. Hout, V1out, V2out, V3out (right end should go inside of left end) $$\begin{array}{|c|c|} \hline 1 & 8 & 2 & 7 \\ \hline 4 & 5 & 3 & 6 \\ \hline \end{array}$$
6. Hout, V1out, V2out, V3out (folded half right end should go in-front of "21") $$\begin{array}{|c|c|} \hline 1 & 8 & 2 & 5 \\ \hline 6 & 7 & 3 & 4 \\ \hline \end{array}$$

6 when fold horizontal 2-nd:

1. V1out, Hout, V2in, V3out (folded half must be put behind "1") $$\begin{array}{|c|c|} \hline 1 & 6 & 5 & 2 \\ \hline 8 & 7 & 4 & 3 \\ \hline \end{array}$$
2. V1out, Hout, V2in, V3in (folded half must be put behind "11") $$\begin{array}{|c|c|} \hline 1 & 6 & 3 & 4 \\ \hline 8 & 7 & 2 & 5 \\ \hline \end{array}$$
3. V1out, Hout, V2out, V3in $$\begin{array}{|c|c|} \hline 1 & 2 & 6 & 7 \\ \hline 4 & 3 & 5 & 8 \\ \hline \end{array}$$
4. V1out, Hout, V2out, V3in (folded half must be put in-front of "21") $$\begin{array}{|c|c|} \hline 1 & 2 & 5 & 6 \\ \hline 8 & 3 & 4 & 7 \\ \hline \end{array}$$
5. V1out, Hout, V2out, V3out $$\begin{array}{|c|c|} \hline 1 & 2 & 8 & 5 \\ \hline 4 & 3 & 7 & 6 \\ \hline \end{array}$$
6. V1out, Hout, V2out, V3out (folded half must be put in-front of "21") $$\begin{array}{|c|c|} \hline 1 & 2 & 7 & 4 \\ \hline 7 & 3 & 6 & 5 \\ \hline \end{array}$$

4 when fold horizontal 3-rd:

1. V1out, V2in, Hout, V3out $$\begin{array}{|c|c|} \hline 1 & 6 & 3 & 8 \\ \hline 8 & 5 & 4 & 7 \\ \hline \end{array}$$
2. V1out, V2in, Hout, V3out (folded half must be put in-front of "21" and "22") $$\begin{array}{|c|c|} \hline 1 & 6 & 3 & 8 \\ \hline 8 & 5 & 4 & 7 \\ \hline \end{array}$$
3. V1out, V2in, Hout, V3in (folded half must be put behind of "11" and "12") $$\begin{array}{|c|c|} \hline 1 & 2 & 5 & 4 \\ \hline 8 & 7 & 6 & 3 \\ \hline \end{array}$$
4. V1out, V2out, Hout, V3in (impossible)
5. V1out, V2out, Hout, V3out (impossible)

4 when fold horizontal 4-th:

1. V1out, V2in, V3out, Hout $$\begin{array}{|c|c|} \hline 1 & 2 & 3 & 4 \\ \hline 8 & 7 & 6 & 5 \\ \hline \end{array}$$
2. Vout, Vin, Vin, Hout $$\begin{array}{|c|c|} \hline 1 & 2 & 4 & 3 \\ \hline 8 & 7 & 5 & 6 \\ \hline \end{array}$$
3. V1out, V2out, V3in, Hout $$\begin{array}{|c|c|} \hline 1 & 4 & 3 & 2 \\ \hline 8 & 5 & 6 & 7 \\ \hline \end{array}$$
4. V1out, V2out, V3out, Hout $$\begin{array}{|c|c|} \hline 1 & 4 & 2 & 3 \\ \hline 8 & 5 & 7 & 6 \\ \hline \end{array}$$

V2 first.

3 when fold horizontal 2-nd:

1. V2out, Hout, V1out, V3out $$\begin{array}{|c|c|} \hline 1 & 8 & 7 & 2 \\ \hline 4 & 5 & 6 & 3 \\ \hline \end{array}$$
2. V2out, Hout, V1out, V3out ("14" goes behind "22") $$\begin{array}{|c|c|} \hline 1 & 8 & 7 & 4 \\ \hline 2 & 3 & 6 & 5 \\ \hline \end{array}$$
3. V2out, Hout, V1out, V3in ("14" goes behind "13") $$\begin{array}{|c|c|} \hline 1 & 8 & 5 & 7 \\ \hline 2 & 3 & 4 & 6 \\ \hline \end{array}$$

6 when fold horizontal 3-nd:

1. V2out, V1***, Hout (is impossible)
2. V2out, V3out, Hout, V1in $$\begin{array}{|c|c|} \hline 1 & 8 & 6 & 7 \\ \hline 2 & 3 & 5 & 4 \\ \hline \end{array}$$
3. V2out, V3in, Hout, V1in $$\begin{array}{|c|c|} \hline 1 & 8 & 7 & 6 \\ \hline 2 & 3 & 4 & 5 \\ \hline \end{array}$$

4. V2in, V1out, Hout, V1in ("14" in-front "13) $$\begin{array}{|c|c|} \hline 1 & 8 & 5 & 4 \\ \hline 2 & 7 & 6 & 3 \\ \hline \end{array}$$

5. V2in, V1out, Hout, V1out ("14" behind "13) $$\begin{array}{|c|c|} \hline 1 & 8 & 3 & 6 \\ \hline 2 & 7 & 4 & 5 \\ \hline \end{array}$$
6. V2out, V3out, Hout, V1in $$\begin{array}{|c|c|} \hline 1 & 6 & 7 & 8 \\ \hline 2 & 5 & 4 & 3 \\ \hline \end{array}$$
7. V2out, V3in, Hout, V1in $$\begin{array}{|c|c|} \hline 1 & 6 & 8 & 7 \\ \hline 2 & 5 & 3 & 4 \\ \hline \end{array}$$

V3 first. H 2-nd

1. V3in, Hout, V1out, V2out

2. V3in, Hout, V1out, V2in

3. V3out, Hout, V1out, V2out

4. V3out, Hout, V1out, V2in

(to be continued)