Happy octopus

This investigation is about happy numbers in the World of the Octopus where all numbers are written in base 8 ... Find all the fixed points and cycles for the happy number sequences in base 8.

Age

11 to 14

Challenge level

Exploring and noticing Working systematically Conjecturing and generalising Visualising and representing Reasoning, convincing and proving

Being curious Being resourceful Being resilient Being collaborative

Problem

Have you met Happy Numbers? Start with any number you like and form a sequence by writing down the sum of the squares of the digits of each number to get the next number in the sequence. For example, from 31 you go to 10 because 3 squared plus 1 squared makes 10. Here are some 'happy' sequences:

31 -> 10 -> 1 -> 1 -> 1 ...

68 -> 100 -> 1 -> 1 -> 1 ...

25 -> 29 -> 85 -> 89 -> 145 -> 42 -> 20 -> 4 -> 16 -> 37 -> 58 -> 89 ->...

122 -> 9 -> 81 -> 65 -> 61 -> 37 ->...

1122 -> 10 -> 1 -> 1 ;...

Numbers are called 'happy' when their sequences, sooner or later, give repeated 1's. We say 1 is a fixed point. The numbers 31, 68 and 1122 are happy and so are the numbers 10 and 100.

The number 25 is sad because, however long we go on with the sequence, it will never come to 1, it will just keep repeating the terms (89, 145, 42, 20, 4, 16, 37, 58) over and over again. We call this an 8-cycle or loop. Can you see why 122 is also a sad number? Can you find some more happy numbers?

This investigation is about happy numbers in the World of the Octopus where all numbers are written in base 8. Octi the octopus counts:

1, 2, 3, 4, 5, 6, 7, 10, 11, 12, 13, 14, 15, 16, 17, 20, 21, 22, 23, 24, 25 ... and so on.

In base 8 the number "one-zero" (written 10) means eight, the number "one-one" means nine etc. and the number "two-zero" (written 20) means two eights or sixteen in base 10. Here are two 'happy' sequences with all numbers written in base 8:

24 -> 24 -> 24 -> 24 -> 24 -> ...

31 -> 12 -> 5-> 31 -> 12 -> 5 ...

We found that twenty, in base 10, that is two-four in base 8, was a sad number. Octi would call two-four a 'happy number' because its sequence 'homes in' on the repeated term (or fixed point) 24.

Three-one, in base 8, is twenty five in base ten; one-two in base 8 is ten in base ten. So, from the sequence above, we can see that three-one, one-two and five are all sad numbers.

Find all the fixed points and cycles for the happy number sequences in base 8.

Student Solutions

This is an investigation of sequences formed by writing down the sum of the squares of the digits of each number to get the next number in the sequence. A number is called happy if it starts a sequence that goes to a number which is repeated over and over again (e.g. 1, 1, 1, 1, $\ldots$) and this is called a fixed point. Other numbers lead to cyclic patterns which repeat over and over again. For example this sequence leads to a repeating 8-cycle:

25, 29, 85, 89, 145, 42, 20, 4, 16, 37, 58, 89, $\ldots$

Rachel Walker from the Mount School, York explained that in base ten, 122 is a sad number because it has an 8-cycle like 25 and this is her solution:

122, 9, 81, 65, 37, 58, 89, 145, 42, 20, 4, 16, 37, 58........

You can find a lot of 'happy numbers' such as: 310, 70, 86, 130, 7, 5555, 1212, 13, 1000 and other numbers which are variations of these, e.g. swapping the numbers the other way around (13, 31) and adding zeros onto the end of numbers (31, 310). Also you get numbers that add up to other happy numbers e.g. 5555 - four fives squared = 100.

The following sequences are the patterns of fixed points and cycles in base eight that I have found. These are fixed points so they are happy numbers and any numbers which start sequences ending like this are happy numbers.

64, 64, 62, 64, 64, $\ldots$

24, 24, 24, 24, 24, $\ldots$

1, 1, 1, 1, ,1, $\ldots$

These are 2-cycles:

32, 15, 32, 15, 32, 15, $\ldots$

20, 4, 20, 4, 20, 4, $\ldots$

This is a 3-cycle:

31, 12, 5, 31, 12, 5, 31, 12, 5, $\ldots$

Pen Areecharoenlert, also from The Mount School, York found some more happy numbers and cycles in base eight. Pen says "There does not appear to be any pattern in which ones are happy, but it looks as though there are 3 fixed points: 1, 24 and 64." This is Pen's list.

1, 1, 1, $\ldots$ so 1 is happy.

2, 4, 20, 4, 20, $\ldots$ so 2 is sad. (2-cycle)

3, 11, 2, 4, 20, $\ldots$ so 3 is sad. (2-cycle)

4 is sad. (2-cycle)

5, 31, 12, 5, $\ldots$ so 5 is sad. (3-cycle)

6, 44, 40, 20, 4, 20, $\ldots$ so 6 is sad. (2-cycle)

7, 61, 45, 51, 32, 15, 32, $\ldots$ so 7 is sad. (2-cycle)

10, 1, 1, 1, 1$\ldots$ so 10 is happy.

11, 2, 4, 20, 4, $\ldots$ so 11 is sad. (2-cycle)

12, 5, 31, 12, $\ldots$ so 12 is sad. (3-cycle)

13, 12, $\ldots$ so 13 is sad. (3-cycle)

14, 21, 5, $\ldots$ so 14 is sad. (3-cycle)

5, 32, 15,$\ldots$ so 15 is sad. (2-cycle)

16, 45, 51, 32, $\ldots$ so 16 is sad. (2-cycle)

17, 62, 50, 31,$\ldots$ so 17 is sad. (3-cycle)

20, 4, 20, 4, $\ldots$ so 20 is sad. (2-cycle)

21, 5, $\ldots$ so 21 is sad. (3-cycle)

22, 10, 1, 1,$\ldots$ so 22 is happy.

23, 15, $\ldots$ so 23 is sad. (2-cycle)

24, 24, 24, 24, so 24 is happy.

25, 35,, 42, 24, 24,$\ldots$ so 25 is happy.

26, 50, 31,$\ldots$ so 26 is sad. (3-cycle)

27, 65, 75, 112, 6, 44, 40, 20,$\ldots$ so 27 is sad. (2-cycle)

30 , 11, 2, 4, 20, $\ldots$ so 30 is sad. (2-cycle)

31, 12, $\ldots$ so 31 is sad. (3-cycle)

32, 15, $\ldots$ so 32 is sad. (2-cycle)

33, 22, 10, 1, $\ldots$ so 33 is happy.

34, 31, 12, $\ldots$ so 34 is sad. (3-cycle)

35, 42, 24, 24, $\ldots$ so 35 is happy.

36, 55, 62, 50, 31, $\ldots$ so 36 is sad. (3-cycle)

37, 72, 65, 75, 112 so 37 is sad. (2-cycle)

40 > 20 > 4 > 20 so 40 is sad. . (2-cycle)

41, 21, 5, 31, 12, 5, $\ldots$ so 41 is sad. (3-cycle)

42, 24, 24,$\ldots$ so 42 is happy.

43, 31, $\ldots$ so 43 is sad. (3-cycle)

44, 40, 20, 4, 20,$\ldots$ so 44 is sad. (2-cycle)

45, 51, 32, $\ldots$ so 45 is sad. (2-cycle)

46, 64, 64, 64, $\ldots$ so 46 is happy.

47, 101, 2, 4, 20, $\ldots$ so 47 is sad. (2-cycle)

Claire Kruithof, Madras College, St Andrews proved that 24 and 64 are the only 2 digit fixed points in base 8. This is Claire's proof:

"We wanted to see if there would be any more fixed points so we used algebra. The number following $ab$ is $a^2 + b^2$ where $a$ and $b$ are between 0 and 7. For a fixed point we have:

\[a^2 + b^2 = 8a + b\] \[b^2 - b = 8a - a^2\] \[b(b - 1) = a (8 - a)\]

Searching for possibilities

B	a
1 x 0 = 0	7 x 1 = 7
2 x 1 = 2	6 x 2 = 12
3 x 2 = 6	5 x 3 = 15
4 x 3 = 12	4 x 4 = 16
5 x 4 = 20
6 x 5 = 30
7 x 6 = 42

The only possibilities are $a = 2, \ b = 4$ and $a = 6, \ b = 4$.

With more digits the sums of squares are never bigger than some multiple of $7^2$. It can be proved that sequences cannot go on for ever without repeating patterns and so all sequences go into cycles or go to a fixed point. Moreover it can be proved that there are no fixed points with 3 or more digits.

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