Conway's Conundrum

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

What is the next term in this sequence?

Hint: Read the numbers aloud!


The answer is 13112221.

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1 solution

Andrei Li
Aug 10, 2018

This sequence is called a " Look & Say " sequence, a type of notational sequence, which obeys the following rule:

1 : 1: Start with a seed value a 0 a_0 .

2 : 2: Read the number of groups of consecutive digits (i.e. 121123 121123 is read "one 1 1 , one 2 2 , two 1 1 s, one 2 2 , one 3 3 ").

3 : 3: Write down the resulting digit (i.e. 1112211223 1112211223 ).

4 : 4: Repeat.

312211 312211 can then be written as "one 3 3 , one 1 1 , two 2 2 s, two 1 1 s", which gives 13112221 13112221 .

Notice that when a 0 = 22 a_0=22 , any output a i = 22 a_i=22 .


Note: This sequence was studied by John Conway, who discovered, among many other characteristics, that when the sequence approached \infty and that the seed value a 0 22 a_0\neq22 , L n + 1 L n \frac{L_{n+1}}{L_n} equaled a constant λ 1.303577269... \lambda \approx 1.303577269... , where L i L_i is the number of digits in the i i th element; λ \lambda is called Conway's constant .

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