Maths Olympiad Prep

Track / Stage 7 / 241 of 300 #1641 of 1964

Problem 1641

National olympiad second round; IMO P1/P4
Number theory Difficulty 7.5 Find the answer

Higher Secondary P7

If there exists a prime number pp such that p+2qp+2q is prime for all positive integer qq smaller than pp, then pp is called an "awesome prime". Find the largest "awesome prime" and prove that it is indeed the largest such prime.

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

Official solution

To find the largest "awesome prime" p p , we need to ensure that p+2q p + 2q is prime for all positive integers q q smaller than p p . Let's analyze the given solution step-by-step.

1. **Verification for p=7 p = 7 **:
- For p=7 p = 7 , we need to check if 7+2q 7 + 2q is prime for all q q such that 1q<7 1 \leq q < 7 .
- Check:
- q=1 q = 1 : 7+2×1=9 7 + 2 \times 1 = 9 (not prime)
- q=2 q = 2 : 7+2×2=11 7 + 2 \times 2 = 11 (prime)
- q=3 q = 3 : 7+2×3=13 7 + 2 \times 3 = 13 (prime)
- q=4 q = 4 : 7+2×4=15 7 + 2 \times 4 = 15 (not prime)
- q=5 q = 5 : 7+2×5=17 7 + 2 \times 5 = 17 (prime)
- q=6 q = 6 : 7+2×6=19 7 + 2 \times 6 = 19 (prime)
- Since 9 9 and 15 15 are not prime, p=7 p = 7 does not satisfy the condition for all q q .

2. **Verification for p=11 p = 11 **:
- For p=11 p = 11 , we need to check if 11+2q 11 + 2q is prime for all q q such that 1q<11 1 \leq q < 11 .
- Check:
- q=1 q = 1 : 11+2×1=13 11 + 2 \times 1 = 13 (prime)
- q=2 q = 2 : 11+2×2=15 11 + 2 \times 2 = 15 (not prime)
- q=3 q = 3 : 11+2×3=17 11 + 2 \times 3 = 17 (prime)
- q=4 q = 4 : 11+2×4=19 11 + 2 \times 4 = 19 (prime)
- q=5 q = 5 : 11+2×5=21 11 + 2 \times 5 = 21 (not prime)
- q=6 q = 6 : 11+2×6=23 11 + 2 \times 6 = 23 (prime)
- q=7 q = 7 : 11+2×7=25 11 + 2 \times 7 = 25 (not prime)
- q=8 q = 8 : 11+2×8=27 11 + 2 \times 8 = 27 (not prime)
- q=9 q = 9 : 11+2×9=29 11 + 2 \times 9 = 29 (prime)
- q=10 q = 10 : 11+2×10=31 11 + 2 \times 10 = 31 (prime)
- Since 15 15 , 21 21 , 25 25 , and 27 27 are not prime, p=11 p = 11 does not satisfy the condition for all q q .

3. **Verification for p=13 p = 13 **:
- For p=13 p = 13 , we need to check if 13+2q 13 + 2q is prime for all q q such that 1q<13 1 \leq q < 13 .
- Check:
- q=1 q = 1 : 13+2×1=15 13 + 2 \times 1 = 15 (not prime)
- q=2 q = 2 : 13+2×2=17 13 + 2 \times 2 = 17 (prime)
- q=3 q = 3 : 13+2×3=19 13 + 2 \times 3 = 19 (prime)
- q=4 q = 4 : 13+2×4=21 13 + 2 \times 4 = 21 (not prime)
- q=5 q = 5 : 13+2×5=23 13 + 2 \times 5 = 23 (prime)
- q=6 q = 6 : 13+2×6=25 13 + 2 \times 6 = 25 (not prime)
- q=7 q = 7 : 13+2×7=27 13 + 2 \times 7 = 27 (not prime)
- q=8 q = 8 : 13+2×8=29 13 + 2 \times 8 = 29 (prime)
- q=9 q = 9 : 13+2×9=31 13 + 2 \times 9 = 31 (prime)
- q=10 q = 10 : 13+2×10=33 13 + 2 \times 10 = 33 (not prime)
- q=11 q = 11 : 13+2×11=35 13 + 2 \times 11 = 35 (not prime)
- q=12 q = 12 : 13+2×12=37 13 + 2 \times 12 = 37 (prime)
- Since 15 15 , 21 21 , 25 25 , 27 27 , 33 33 , and 35 35 are not prime, p=13 p = 13 does not satisfy the condition for all q q .

4. **Verification for p=19 p = 19 **:
- For p=19 p = 19 , we need to check if 19+2q 19 + 2q is prime for all q q such that 1q<19 1 \leq q < 19 .
- Check:
- q=1 q = 1 : 19+2×1=21 19 + 2 \times 1 = 21 (not prime)
- q=2 q = 2 : 19+2×2=23 19 + 2 \times 2 = 23 (prime)
- q=3 q = 3 : 19+2×3=25 19 + 2 \times 3 = 25 (not prime)
- q=4 q = 4 : 19+2×4=27 19 + 2 \times 4 = 27 (not prime)
- q=5 q = 5 : 19+2×5=29 19 + 2 \times 5 = 29 (prime)
- q=6 q = 6 : 19+2×6=31 19 + 2 \times 6 = 31 (prime)
- q=7 q = 7 : 19+2×7=33 19 + 2 \times 7 = 33 (not prime)
- q=8 q = 8 : 19+2×8=35 19 + 2 \times 8 = 35 (not prime)
- q=9 q = 9 : 19+2×9=37 19 + 2 \times 9 = 37 (prime)
- q=10 q = 10 : 19+2×10=39 19 + 2 \times 10 = 39 (not prime)
- q=11 q = 11 : 19+2×11=41 19 + 2 \times 11 = 41 (prime)
- q=12 q = 12 : 19+2×12=43 19 + 2 \times 12 = 43 (prime)
- q=13 q = 13 : 19+2×13=45 19 + 2 \times 13 = 45 (not prime)
- q=14 q = 14 : 19+2×14=47 19 + 2 \times 14 = 47 (prime)
- q=15 q = 15 : 19+2×15=49 19 + 2 \times 15 = 49 (not prime)
- q=16 q = 16 : 19+2×16=51 19 + 2 \times 16 = 51 (not prime)
- q=17 q = 17 : 19+2×17=53 19 + 2 \times 17 = 53 (prime)
- q=18 q = 18 : 19+2×18=55 19 + 2 \times 18 = 55 (not prime)
- Since 21 21 , 25 25 , 27 27 , 33 33 , 35 35 , 39 39 , 45 45 , 49 49 , 51 51 , and 55 55 are not prime, p=19 p = 19 does not satisfy the condition for all q q .

5. **Verification for p=5 p = 5 **:
- For p=5 p = 5 , we need to check if 5+2q 5 + 2q is prime for all q q such that 1q<5 1 \leq q < 5 .
- Check:
- q=1 q = 1 : 5+2×1=7 5 + 2 \times 1 = 7 (prime)
- q=2 q = 2 : 5+2×2=9 5 + 2 \times 2 = 9 (not prime)
- q=3 q = 3 : 5+2×3=11 5 + 2 \times 3 = 11 (prime)
- q=4 q = 4 : 5+2×4=13 5 + 2 \times 4 = 13 (prime)
- Since 9 9 is not prime, p=5 p = 5 does not satisfy the condition for all q q .

6. **Verification for p=3 p = 3 **:
- For p=3 p = 3 , we need to check if 3+2q 3 + 2q is prime for all q q such that 1q<3 1 \leq q < 3 .
- Check:
- q=1 q = 1 : 3+2×1=5 3 + 2 \times 1 = 5 (prime)
- q=2 q = 2 : 3+2×2=7 3 + 2 \times 2 = 7 (prime)
- Since both 5 5 and 7 7 are prime, p=3 p = 3 satisfies the condition for all q q .

Therefore, the largest "awesome prime" is p=3 p = 3 .

The final answer is 3 \boxed{3} .

Source: NuminaMath-1.5, licensed Apache-2.0. Statement and solution reproduced as published; topic, difficulty and ordering added by this site.