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

Composite numbers are opposite to prime numbers.

• ...,
• 514,
• 515,
• 516,
• 517,
• 518,
• 519,
• 520,
• ...

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.

• ...,
• 513,
• 514,
• 515,
• 517,
• 518,
• 519,
• 521,
• ...

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.

• ...,
• 489,
• 495,
• 511,
• 517,
• 519,
• 529,
• 535,
• ...

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.

• ...,
• 511,
• 513,
• 515,
• 517,
• 519,
• 521,
• 523,
• ...

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

Guess what evil numbers are.

• ...,
• 511,
• 512,
• 515,
• 517,
• 518,
• 521,
• 522,
• ...

A composite number is called a Smith number if the sum of its digits equals the sum of all the digits appearing in its prime divisors (counting multiplicity).

In 1984, when Albert Wilansky called his brother-in-law, named Smith, he noticed that the phone number possesses the property described here. Are they called joke numbers, because they were named after an innocent unsuspecting brother-in-law :-) ?

• ...,
• 438,
• 454,
• 483,
• 517,
• 526,
• 535,
• 562,
• ...

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

• ...,
• 511,
• 514,
• 515,
• 517,
• 518,
• 519,
• 521,
• ...