A string (and, in fact, the *only* string,) of 10 single-digit numbers, `d`_{9}`d`_{8}`d`_{7}`d`_{6}…`d`_{0}, such that `d`_{n} is the number of occurrences of the digit `n` in the string. It is sometimes written 2100010006 (`d`_{1}`d`_{2}`d`_{3}…`d`_{9}`d`_{0}).

It is interesting to note that there is no corresponding string in bases 2, 3, or 6. There are **two** strings in base 4, 0121 and 0202. In bases 5, 7, 8, and 9, there's one string each: 00212, 0001123, 00010124, 000100125.

In base `b`, `b` ≥ 7, there is a regular pattern: "0001", then (7 - `b`) "0"s, then "12", then (`b` - 4). These are the *only* patterns for `b` between 7 and 50 inclusive, and may be the only type for *any* base greater than 6.