The integer sequence beginning with a single digit in which the next term is obtained by describing the previous term. Starting
with 1, the sequence would be defined by "1, one 1, two 1s, one 2 one 1,"
etc., and the result is 1, 11, 21, 1211, 111221, .... Similarly, starting the sequence
instead with the digit 
for 
gives 
, 1
, 111
, 311
, 13211
, 111312211
, 31131122211
, 1321132132211
, ..., as summarized in the following table.
 | OEIS | sequence |
| 1 | A005150 | 1, 11, 21, 1211, 111221, 312211, 13112221, 1113213211, ... |
| 2 | A006751 | 2, 12, 1112, 3112, 132112, 1113122112, 311311222112, ... |
| 3 | A006715 | 3, 13, 1113, 3113, 132113, 1113122113, 311311222113, ... |
The number of digits in the 
th term of the sequence for 
are 1, 2, 2, 4, 6, 6, 8, 10, 14, 20, 26, 34, 46, 62, ...
(OEIS A005341). Similarly, the numbers of digits
for the 
th
term of the sequence for 
, 3, ..., are 1, 2, 4, 4, 6, 10, 12, 14, 22, 26, ... (OEIS
A022471). These sequences are asymptotic to
,
where
 |  |  |
(1)
|
 |  |  |
(2)
|
 |  |  |
(3)
|
The quantity 
is known as Conway's constant (OEIS A014715),
and amazingly is given by the unique positive real root of the polynomial
 |
(4)
|
all of whose roots are illustrated above.
In fact, the constant is even more general than this, applying to all starting sequences (i.e., even those starting with arbitrary starting digits), with the exception of 22, a result which follows from the cosmological theorem. Conway discovered that strings sometimes factor as a concatenation of two strings whose descendants never interfere with one another. A string with no nontrivial splittings is called an "element," and other strings are called "compounds." It is postulated that every string of 1s, 2s, and 3s that does not contain four of the same number in succession eventually "decays" into a compound of 92 special elements, named after the chemical elements.
