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

Composite numbers are opposite to prime numbers.

• ...,
• 216,
• 217,
• 218,
• 219,
• 220,
• 221,
• 222,
• ...

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.

• ...,
• 215,
• 217,
• 218,
• 219,
• 221,
• 223,
• 225,
• ...

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

Guess what odious numbers are.

• ...,
• 212,
• 215,
• 216,
• 219,
• 221,
• 222,
• 225,
• ...

One can take the sum of the squares of the digits of a number. Those numbers are happy for which iterating this operation eventually leads to 1.

• ...,
• 193,
• 203,
• 208,
• 219,
• 226,
• 230,
• 236,
• ...

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.

• ...,
• 201,
• 205,
• 211,
• 219,
• 223,
• 231,
• 235,
• ...

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.

• ...,
• 213,
• 215,
• 217,
• 219,
• 221,
• 223,
• 225,
• ...

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

• ...,
• 215,
• 217,
• 218,
• 219,
• 221,
• 222,
• 223,
• ...

The next Ulam number is uniquely the sum of two earlier distinct Ulam numbers.

• ...,
• 197,
• 206,
• 209,
• 219,
• 221,
• 236,
• 238,
• ...