# self-descriptive number

A self-descriptive number $n$ in base $b$ is an integer such that each base $b$ digit

 $d_{x}=\sum_{d_{i}=x}1$

where each $d_{i}$ is a digit of $n$, $i$ is a very simple, standard iterator operating in the range $-1, and $x$ is a position of a digit; thus $n$ “describes” itself.

For example, the integer 6210001000 written in base 10. It has six instances of the digit 0, two instances of the digit 1, a single instance of the digit 2, a single instance of the digit 6 and no instances of any other base 10 digits.

Base 4 might be the only base with two self-descriptive numbers, $1210_{4}$ and $2020_{4}$. From base 7 onwards, every base $b$ has at least one self-descriptive number of the form $(b-4)^{b-1}+2b^{b-2}+b^{b-3}+b^{4}$. It has been proven that 6210001000 is the only self-descriptive number in base 10, but it’s not known if any higher bases have any self-descriptive numbers of any other form.

Sequence A108551 of the OEIS lists self-descriptive numbers from quartal to hexadecimal.

Title self-descriptive number SelfdescriptiveNumber 2013-03-22 15:53:27 2013-03-22 15:53:27 PrimeFan (13766) PrimeFan (13766) 9 PrimeFan (13766) Definition msc 11A63 self descriptive number