Prove that there is no positive integer such that, for , the leftmost digit (in decimal notation) of equals .
Solution
1. Assume for contradiction: Suppose there exists a positive integer such that for , the leftmost digit of equals .
2. Define the reduced copy: Let the reduced copy of a positive integer be , where for a nonnegative integer and a terminating decimal such that .
3. Leading digit condition: If the leading digit of is , then the reduced copy of is between and .
4. Analyze the reduced copy of factorials:
- The reduced copy of is between and .
- The reduced copy of is between and .
5. Generalize the bounds:
- For , the reduced copy must be between and .
- For , the reduced copy must be between and .
- For , the reduced copy must be between and .
6. Establish the range: Note that the lower bound is always and the upper bound starts at and slowly reduces to .
7. Claim: The reduced copy of all numbers from to must be between and .
8. Proof by contradiction:
- Suppose not. Since the numbers from to are consecutive, there will be a number from to with a reduced copy between and .
- This is impossible because that does not fit into any of the earlier ranges (which had a lower bound of and an upper bound of at most ).
9. Contradiction:
- Note that the reduced copy of is between and .
- Since the reduced copies of are all between and , the reduced copy of is less than .
- This is a contradiction as it must be between and .
Therefore, there is no positive integer such that for , the leftmost digit of equals .