Look and say... forever?

Given the look and say sequence with a seed of 1,

1 , 11 , 21 , 1211 , 111221 , 312211 , 13112221 , 1113213211 , . . . 1, 11, 21, 1211, 111221, 312211, 13112221, 1113213211, ...

Is it true that this is a strictly increasing sequence?

Notes:

  1. The sequence is in base 10 10 . To prove for a general base N N may be extremely difficult

  2. This may help: Cosmological Theorem

  3. This may also help: Look and Say Sequence

False True

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3 solutions

Sorry about the sloppy writing, there was just too much to write about. I hope it serves as a general idea.

Facts:

1- there can only be runs of length 3 or less of any digit, because of the nature of the dynamical rule. To understand, if we have 2222 2222 as part of the element a n a_n of the sequence, it means that the corresponding part in a n 1 a_{n-1} has been 2222 2222 , which should be translate into 42 42 rather than 2222 2222 .

As a consequence digits of the elements of the sequence can only be { 1 , 2 , 3 } \{1,2,3\} , given the seed is 1 1 .

2- If we index the digits of a number as a 0 a 1 a t a_0a_1 \dots a_t , then a run of length 3 3 can only start at an even position.

3- runs of length 1 , 2 , 3 1,2,3 in a n a_n would cause the following number digits in a n + 1 a_{n+1} : 2 , 2 , 2 2,2,2 .

4- No element a n a_n would start with a run of length 2 2 or 3 3 of the digits 2 2 or 3 3 . Because, if there is such an element, going backwards in the sequence, we would always have elements that would start with 2 2 or 3 3 , but obviously the seed is 1 1 (starts with 1).

Now we put facts together. If a n a_n consists of only runs of length 1, the obviously a n < a n + 1 a_n <a_{n+1} .

If a n a_n consists of both runs of length 1 and 2 and no run of length 3, then the number of digits of a n + 1 a_{n+1} is more than the number of digits a n a_n , hence a n < a n + 1 a_n < a_{n+1} .

If a n a_n consists only of runs of length 2,the length is maintained, but it should start with 11 11 which leads to a n + 1 a_{n+1} starting with 21 21 , so a n < a n + 1 a_{n} < a_{n+1} .

If a n a_{n} contains any run of length 3, then it would be followed by a run of length 1 1 , which automatically compensate the reduction of the number of digits in a n + 1 a_{n+1} that is caused by the run of length 3 3 . So, the number of digits of a n + 1 a_{n+1} would not be less than the number of digits of a n a_n . Now, in case a n a_n and a n + 1 a_{n+1} have the same number of digits, if a n a_n starts with 11 11 , then a n < a n + 1 a_n < a_{n+1} , for a n + 1 a_{n+1} start with 21 21 . Note that if it starts with a run of 1 1 , then the number of digits of a n + 1 a_{n+1} would be definitely more than the number digits of a n a_n .

Arka Dutta
Mar 21, 2019

It's stating the no. Of particular digit followed by the digit itself stated in the previous no. Like 1 and then one 1 = 11 and then two 1=21.... So it's ever expanding

Shuvodip Das
Feb 14, 2019

This look and say sequence is always a increasing one as for you need to say how many numbers are there in the previous term to get the next one.And so if you have 2 digit in term then you will get at least 2 or 4 digits in the next term.If you get 2 digit then those 2 digits will never be same in the next term and will form out as a 4 digital term.And so the answer is TRUE

What if, for example, I start with '111'? This would then turn into '31' which is a smaller number.

Timothy Cao - 2 years, 3 months ago

Actually the look and say sequence starts with one digit. At least the one in the question did

Shuvodip Das - 2 years, 3 months ago

This is just an example, what if at some point you get a number such as 111222111? this would turn into 313231 which is smaller. This is once again just one example of how your statement is not necessarily true. Even though the answer is indeed that it is always increasing, your reasoning is not rigorous and has too many assumptions to prove anything.

Timothy Cao - 2 years, 3 months ago

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Sorry Brother but can I know from where did you get the example 111222111.I mean what's it's previous term? Just my curiosity! Can we ever get 3 same digits consecutively in a look and say sequence? I mean Can WE EVER?Unless it's the first term?

Shuvodip Das - 2 years, 3 months ago

One way to prove it is to show that such a number will never exist, or can exist but will never make the number smaller more than another part of the number will make it bigger.

Timothy Cao - 2 years, 3 months ago

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I'm just using logic to solve problems, brother, sowing seed of 1. Dull people like us needs logic to understand. Brilliant people like you understands equations.(No offense though)

Shuvodip Das - 2 years, 3 months ago

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