Problem:
How many positive integers less than or equal to can be expressed as a sum of distinct factorials? Consider and to be distinct.
Problem:
How many positive integers less than or equal to can be expressed as a sum of distinct factorials? Consider and to be distinct.
Solution:
Answer:
Note that , , , and . These are the only numbers less than that can be written as the sum of factorials. The only other factorials less than are , , and . So a positive integer less than or equal to can only contain , , , and/or one of , or in its sum. If it contains any factorial larger than , it will be larger than . So a sum less than or equal to will either include or not ( ways), or not ( ways), or not ( ways), and add an additional or ( ways). This gives integers less than . However, we want only positive integers, so we must not count . So there are such positive integers.