A positive integer greater than 1 that is not prime is called composite.

Composite numbers are opposite to prime numbers.

• ...,
• 28,
• 30,
• 32,
• 33,
• 34,
• 35,
• 36,
• ...

The number n is deficient if the sum of all its positive divisors except itself is less than n.

Compare with perfect and abundant numbers.

• ...,
• 29,
• 31,
• 32,
• 33,
• 34,
• 35,
• 37,
• ...

The number n is evil if it has an even number of 1's in its binary expansion.

Guess what odious numbers are.

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• 27,
• 29,
• 30,
• 33,
• 34,
• 36,
• 39,
• ...

To build the lucky number sequence, start with natural numbers. Delete every second number, leaving 1, 3, 5, 7, 9, 11, 13, 15, 17, 19, 21, ... . The second number remaining is 3, so delete every third number, leaving 1, 3, 7, 9, 13, 15, 19, 21, ... . The next number remaining is 7, so delete every 7th number, leaving 1, 3, 7, 9, 13, 15, 21, ... . The next number remaining is 9, so delete every ninth number, etc.

Those numbers were lucky they weren't crossed out.

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• 21,
• 25,
• 31,
• 33,
• 37,
• 43,
• 49,
• ...

A number is odd if it is not divisible by 2.

Numbers that are not odd are even. Compare with another pair -- evil and odious numbers.

• ...,
• 27,
• 29,
• 31,
• 33,
• 35,
• 37,
• 39,
• ...

A palindrome is a number that reads the same forward or backward.

• ...,
• 9,
• 11,
• 22,
• 33,
• 44,
• 55,
• 66,
• ...

A number is said to be square-free if its prime decomposition contains no repeated factors.

• ...,
• 29,
• 30,
• 31,
• 33,
• 34,
• 35,
• 37,
• ...