Call a positive integer $n$ "Fantastic" if none of its digits are zero and it is possible to remove one of its digits and reach to an integer which is a divisor of $n$ . ( for example , 25 is fantastic , as if we remove digit 2 , resulting number would be 5 which is divisor of 25 ) Prove that the number of Fantastic numbers is finite.
Problem
Source: 2021 Iran second round mathematical Olympiad P2
Tags: number theory, Digits
15.05.2021 13:11
15.05.2021 21:44
Your statement is right, but the argument here is a little flawed (for example $35 | 385$). It's easy to fix though. Tintarn wrote: Second case: We remove a digit which is neither the last nor the first. So write $n=10^{k+1}a+10^kb+c$ with $a>0$ and where $0<c<10^k$ and $1 \le b \le 9$ is the digit we remove. So $10^ka+c \mid n$ but it also divides $10^{k+1}a+10c$, hence $10^ka+c \mid 10^kb-9c$. But the RHS is not a multiple of $10$, hence in particular non-zero. On the other hand, its absolute value is clearly less than $10^{k+1}$ while $10^{k+1}a+10c>10^{k+1}$. Contradiction! (Note that here we did not use anything on the size of $k$.)