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PrintSAUDI ARABIAN MATHEMATICAL COMPETITIONS
Saudi Arabia number theory
Problem
How many integers satisfy the following conditions? i) , ii) there exist such that and is divisible by all integers from to , except two numbers and .
Solution
The answer is .
We can see that if for some integers and then which implies that , , then , contradiction.
Hence and must be the powers of primes. But one of these numbers is even so one of them must be the power of . On the other hand, must be less than , otherwise leads to , contradiction. With the existence of we can easily choose .
Thus, number satisfies the given condition if and only if there exists an exponent of less than and bigger than , namely such that or is a power of some prime. We can check directly each range of numbers:
1. For each number we can choose , .
2. For each number we can choose and .
3. For each number from we cannot choose any since and are not the powers of prime.
4. For each number from we cannot choose any since and are not powers of prime.
Therefore, the total number of integers we need to find is .
We can see that if for some integers and then which implies that , , then , contradiction.
Hence and must be the powers of primes. But one of these numbers is even so one of them must be the power of . On the other hand, must be less than , otherwise leads to , contradiction. With the existence of we can easily choose .
Thus, number satisfies the given condition if and only if there exists an exponent of less than and bigger than , namely such that or is a power of some prime. We can check directly each range of numbers:
1. For each number we can choose , .
2. For each number we can choose and .
3. For each number from we cannot choose any since and are not the powers of prime.
4. For each number from we cannot choose any since and are not powers of prime.
Therefore, the total number of integers we need to find is .
Final answer
292
Techniques
Prime numbersGreatest common divisors (gcd)Factorization techniques