Look-and-say sequence
Integer sequence / From Wikipedia, the free encyclopedia
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In mathematics, the look-and-say sequence is the sequence of integers beginning as follows:
- 1, 11, 21, 1211, 111221, 312211, 13112221, 1113213211, 31131211131221, ... (sequence A005150 in the OEIS).
To generate a member of the sequence from the previous member, read off the digits of the previous member, counting the number of digits in groups of the same digit. For example:
- 1 is read off as "one 1" or 11.
- 11 is read off as "two 1s" or 21.
- 21 is read off as "one 2, one 1" or 1211.
- 1211 is read off as "one 1, one 2, two 1s" or 111221.
- 111221 is read off as "three 1s, two 2s, one 1" or 312211.
The look-and-say sequence was analyzed by John Conway[1] after he was introduced to it by one of his students at a party.[2][3]
The idea of the look-and-say sequence is similar to that of run-length encoding.
If started with any digit d from 0 to 9 then d will remain indefinitely as the last digit of the sequence. For any d other than 1, the sequence starts as follows:
- d, 1d, 111d, 311d, 13211d, 111312211d, 31131122211d, …
Ilan Vardi has called this sequence, starting with d = 3, the Conway sequence (sequence A006715 in the OEIS). (for d = 2, see OEIS: A006751)[4]