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

Composite numbers are opposite to prime numbers.

• ...,
• 640,
• 642,
• 644,
• 645,
• 646,
• 648,
• 649,
• ...

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.

• ...,
• 639,
• 641,
• 643,
• 645,
• 646,
• 647,
• 649,
• ...

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

Guess what odious numbers are.

• ...,
• 639,
• 640,
• 643,
• 645,
• 646,
• 649,
• 650,
• ...

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.

• ...,
• 631,
• 639,
• 643,
• 645,
• 651,
• 655,
• 673,
• ...

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.

• ...,
• 639,
• 641,
• 643,
• 645,
• 647,
• 649,
• 651,
• ...

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 :-) ?

• ...,
• 627,
• 634,
• 636,
• 645,
• 648,
• 654,
• 663,
• ...

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

• ...,
• 641,
• 642,
• 643,
• 645,
• 646,
• 647,
• 649,
• ...