Problem:
Find the largest integer less than all of whose divisors have at most two 's in their binary representations.
Solution
Solution:
Call a number good if all of its positive divisors have at most two 's in their binary representations. Then, if is an odd prime divisor of a good number, must be of the form . The only such primes less than are , and , so the only possible prime divisors of are , and .
Next, note that since , if either or is greater than , then there will be at least 's in the binary representation of , so cannot divide a good number. On the other hand, if , then , so is a good number and can divide a good number. Finally, note that since multiplication by in binary just appends additional 's, if is a good number, then is also a good number.
It therefore follows that any good number less than must be of the form , where belongs to (and moreover, all such numbers are good). It is then straightforward to check that the largest such number is .