Let us call a positive integer [i]pedestrian[/i] if all its decimal digits are equal to 0 or 1. Suppose that the product of some two pedestrian integers also is pedestrian. Is it necessary in this case that the sum of digits of the product equals the product of the sums of digits of the factors?
Solution
Let us consider the definition of a pedestrian integer: a positive integer whose decimal digits are all either 0 or 1. As per the problem statement, we need to determine if, when the product of two pedestrian integers is itself pedestrian, the sum of the digits of the product necessarily equals the product of the sums of the digits of the factors.
To explore this, consider two pedestrian integers and , and their product .
### Example
Let (whose digits are 1 and 0) and (whose digits are 1 and 1).
Now, compute the product:
Here, the digits of are 1, 1, and 0, verifying that is pedestrian.
### Sum of Digits
The sum of the digits of is:
The sum of the digits of is:
The sum of the digits of the product is:
### Product of Sums
Now compute the product of the sums of the digits of the factors:
### Comparison
For this example, we see that:
However, let us further analyze if it's necessary for this equality to hold in general by testing different combinations.
### Counterexample
Consider and .
Digits of are 1, 0, and 0, thus is pedestrian.
Sum of the digits of is:
Sum of the digits of and are both:
Product of the sums:
In this example, and . While they match, this example reveals no counterexample against the initial observation. However, considering other configurations or operations may yield different results as illustrated initially.
### Conclusion
The equality might hold for specific examples, but it doesn't establish a necessary rule for all eventualities, especially with higher combinations or potential operations where the sum of digits differs. Thus, based on the analysis and logical exploration:
The answer is: