The integer is the smallest positive integer that is a multiple of 2024, has more than 100 positive divisors (including 1 and ), and has fewer than 110 positive divisors (including 1 and ). What is the sum of the digits of ?
Solution
Throughout this solution, we use the fact that if is a positive integer with and has prime factorization for some distinct prime numbers and positive integers , then the number of positive divisors of including 1 and is equal to . We are told that is a positive multiple of 2024. Now, . This means that has at least 3 prime factors (namely 2,11 and 23) and that at least one of these prime factors has an exponent of at least 3. Let be the number of positive divisors that has. We are told that . Since and has at least 3 prime factors, then is a positive integer that can be written as the product of at least 3 positive integers each greater than 2. cannot equal , or 109, since each of these is prime (and so cannot be written as the product of 3 integers each at least 2). also cannot equal 106 because (both 2 and 53 are prime), which means that 106 cannot be written as the product of three integers each greater than 1. The possible values of that remain are . We note that and and and . Case 1: Since the prime factors of include at least 2,11 and 23, then the prime factorization of includes factors of and for some positive integers and with . If a fourth prime power was also a factor of , then would be divisible by could have more factors if had more prime factors.) Since has only 3 prime factors, it cannot be written as the product of 4 integers each greater than 1. Thus, cannot have a fourth prime factor. This means that , which gives . This means that and are equal to 2,3 and 17, in some order, and so and are equal to 1,2 and 16, in some order. For to be as small as possible, the largest exponent goes with the smallest prime, the next largest exponent with the next smallest prime, and so on. (Can you see why this makes as small as possible?) Therefore, the smallest possible value of in this case is . Case 2: Using a similar argument, we can determine that with and equal to 3,5 and 7 in some order, meaning that and equal in some order. Therefore, the minimum value of is this case is . Case 3: Since has 4 prime factors, then cannot have more than 4 prime factors. (If had 5 or more prime factors, then the product equal to would include at least 5 integers, each at least 2.) Therefore, and , or for some prime and . This means that or . In the case that has three prime factors, we note that are the only two ways of writing 104 as the product of 3 integers each of which is at least 2. These give corresponding minimum values of and . In the case that has four prime factors, then means that and are in some order. This in turn means that the corresponding smallest possible value of is . We note here that the prime power has become in order to minimize both (since ) and its exponent. Case 4: Since has 5 prime factors, then cannot have more than 5 prime factors. If has 5 prime factors, then we need to use the factorization . This is not possible, however, because the power must have which would mean that one of the five factors of would have to be at least 4. If has 4 prime factors, then must be partitioned as or or . (Since two of the prime factors have to be combined, either two 2 s , two 3 s , or a 2 and a 3 are combined.) These give minimum values of and and . If has 3 prime factors, then we must use one of the factorizations or or or or . These gives corresponding minimum values , , , , . Combining Cases 1 through 4, the minimum possible value of is 582912. The sum of the digits of 582912 is .