Is 1 a prime number?
1 is neither prime nor composite, because a prime needs exactly two factors and 1 has one.
Checking 1 for primality, divisor by divisor
1 is not tested: by definition a prime has exactly two factors, and 1 has only one.
You only need to test primes up to the square root of 1, about 1: if 1 had a factor bigger than that, its partner would be smaller and would already have been found.
The quick checks first
- Divisible by 2? No: the last digit is odd.
- Divisible by 3? No: the digit sum is 1.
- Divisible by 5? No: the last digit is 1.
- Divisible by 9? No: the digit sum is 1.
- Divisible by 10? No: the last digit is 1.
All the factors of 1
1
The primes near 1
The prime before 1 is none; the prime after it is 2.
🧮 Calculator
The answer appears here, with the working in words.
✏️ Five questions, answers hidden
1. Is 2 prime?
Answer: yes
2. Is 3 prime?
Answer: yes
3. What is the smallest prime factor of 5?
Answer: 5
4. List the primes between 10 and 30.
Answer: 11, 13, 17, 19, 23, 29
5. Up to which number do you need to test divisors for 4?
Answer: the square root, about 2
📚 The class this belongs to
Prime and composite numbers are taught in Grade 4 and 5 in the US, Year 5 in the UK (KS2), Class 5 and 6 in CBSE and Year 5 and 6 in Australia.
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