# Numbers Equal to Number of Digits in Factorial

## Theorem

For $n \in \set {1, 22, 23, 24}$ the number of digits in the decimal representation of $n!$ is equal to $n$.

## Proof

First we note that:

- $1! = 1$

which has $1$ digit.

Then from Examples of Factorials:

- $2! = 2$

which has $1$ digit.

Multiplying $n$ by a $1$-digit number increases the number of digits of $n$ by no more than $1$.

Thus from $n = 3$ to $n = 9$, $n!$ has no more than $n - 1$ digits.

From $21$ Factorial:

- $21! = 51 \, 090 \, 942 \, 171 \, 709 \, 440 \, 000$

and so $21!$ has $20$ digits.

Multiplying $n$ by a $2$-digit number increases the number of digits of $n$ by at least $1$.

Thus from $n = 10$ to $n = 20$, $n!$ has no more than $n - 1$ digits.

It is noted that:

- From $22$ Factorial:
- $22! = 1 \, 124 \, 000 \, 727 \, 777 \, 607 \, 680 \, 000$

- which has $22$ digits.

- From $23$ Factorial:
- $23! = 25 \, 852 \, 016 \, 738 \, 884 \, 976 \, 640 \, 000$

- which has $23$ digits.

- From $24$ Factorial:
- $24! = 620 \, 448 \, 401 \, 733 \, 239 \, 439 \, 360 \, 000$

- which has $24$ digits.

But from $25$ Factorial:

- $25! = 15 \, 511 \, 210 \, 043 \, 330 \, 985 \, 984 \, 000 \, 000$

which has $26$ digits.

As above, multiplying $n$ by a number with $2$ digits or more increases the number of digits of $n$ by at least $1$.

So if $n!$ has more than $n$ digits, $\paren {n + 1}!$ has more than $n + 1$ digits.

It follows by induction that for all $n \ge 25$, $n!$ has more than $n$ digits.

Hence the result.

$\blacksquare$

## Sources

- 1986: David Wells:
*Curious and Interesting Numbers*... (previous) ... (next): $22$ - 1997: David Wells:
*Curious and Interesting Numbers*(2nd ed.) ... (previous) ... (next): $22$