What is is 21 and 17 weird?

21 and 17 are considered "weird" numbers because of a specific property:

• A number is considered weird if it is https://www.wikiwhat.page/kavramlar/abundant%20number (the sum of its proper divisors is greater than the number itself), but not https://www.wikiwhat.page/kavramlar/perfect%20number (the sum of its proper divisors equals the number) and cannot be expressed as the sum of some of its proper divisors.

Let's examine 21:

• Proper divisors of 21: 1, 3, 7
• Sum of proper divisors: 1 + 3 + 7 = 11
• 11 < 21, so 21 is not abundant and, therefore, not weird.

Now let's examine 17:

• Proper divisors of 17: 1
• Sum of proper divisors: 1
• 1 < 17, so 17 is not abundant and, therefore, not weird.

Therefore, neither 21 nor 17 is a weird number.

Essentially, to be considered weird, a number must first be abundant and then fail a further condition regarding the sum of its divisors.