Maths Olympiad Prep

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Number theory Difficulty 6.3 National olympiad Find the answer

We call a number greater than 2525, semi-prime if it is the sum of some two different prime numbers. What is the greatest number of consecutive natural numbers that can be semi-prime?

A number or a short expression. Spacing and $ signs are ignored.

Solution

To solve this problem, we need to find the greatest number of consecutive natural numbers greater than 25 that can each be represented as the sum of two different prime numbers, which we refer to as semi-prime.

First, let's identify a few prime numbers as potential candidates: 2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, etc.

To check if a number n n is semi-prime, we look for two distinct primes p1 p_1 and p2 p_2 such that:
n=p1+p2 n = p_1 + p_2

We systematically check consecutive numbers starting from 26 (since the problem states the number must be greater than 25) to form pairs that sum to these numbers.

Let's test some consecutive numbers:

- 26: 26=23+3 26 = 23 + 3 (both are primes)
- 27: 27=19+7 27 = 19 + 7 (both are primes)
- 28: 28=23+5 28 = 23 + 5 (both are primes)
- 29: 29=23+3 29 = 23 + 3 (both are primes)
- 30: 30=23+7 30 = 23 + 7 (both are primes)
- 31: 31=29+2 31 = 29 + 2 (both are primes)

We observe that from 26 to 31, all numbers are semi-prime, providing a total of 6 consecutive numbers.

Let's check the next sequence:

- 32: 32=29+3 32 = 29 + 3 (both are primes)
- 33: 33=31+2 33 = 31 + 2 (both are primes)
- ... continue this checking process

After further inspection, we notice that the sequence length where each number is semi-prime stabilizes at 5 for most sets of consecutive numbers checked beyond 26 to 31.

Therefore, the greatest number of consecutive natural numbers greater than 25 that can be semi-prime is:
5 \boxed{5}

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