[Math] Are there numbers that describe themselves in some base but not according to the patttern $6210001000$

decimal-expansionnumber theory

Call the first digit of a number digit $0$. The digit after that digit $1$, and so on and so forth.

In base $10$, the number $6210001000$ describes itself, because digit $0$ is $6$ and it has six $0$s. Digit $1$ is $2$ and it has $2$ $1s$. Digit $2$ is $1$ and there is only one $2$. There are none of the other numerals.

So it occurs to me that in duodecimal, you can do $821000001000$, which is $6073061476032$ in decimal. In base $40,$ you could do $Z21000…$ something like that, you get the idea. You could keep going, the only problem being not having enough symbols.

According to a graphic I saw on Facebook, $6210001000$ is the only such number in base $10$. I'm wondering: might there be some base in which a number describes itself like this but its first digit isn't 4 less than the base, followed by $21$ and a bunch of 0s with a 1 close to the end?

Best Answer

For bases sufficiently large (as @Goos noted, there are examples for low bases), it is impossible. I will use the facts that there must be as many numbers as the base, and that the digits must sum to the base (since they count the digits in the number).

Take the base $=n$, and consider the first digit $k$. $k$ cannot be zero, because that would say there are zero zero's, which there wouldn't be! So there are $k$ zero's, and at least one $k$ (for now), meaning we have $$n-k-1$$ digits left to fill. These digits must add to the remaining $n-k-k\cdot 0=n-k$, and the only $n-k-1$ digits which sum to $n-k$ are $$1,1,\ldots,1,2$$ (we have $n-k-2$ one's). I will eliminate $k=1,2$ later. So we have $2$ of something, not $k$'s and not zero's. We must have $2$ one's. And then we have one $k$ and one $2$, which works. The condition that the numbers sum to the base gives $$k+1+1+2+k\cdot 0=k+4=n \implies k=n-4$$ so the only possible self-descriptive number is $$(n-4)(2)(1)0\ldots(1)0\ldots0.$$


If $k=1$, then there will have to be two one's, and one two, and one zero. This gives the only possibility $$(1210)_4$$ in base $4$.

If $k=2$, then there must be two two's. We have two zero's, so there are one or no one's. The two possibilities here are $$(21200)_5,\quad (2020)_4$$ So we have enumerated all self-descriptive numbers.

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