11 (eleven) is the natural number following 10 and preceding 12. It is the first repdigit. 11 is a prime number. It is the smallest two-digit prime number in the decimal base.

#### Natural Numbers 0-20

In mathematics, natural numbers are those used for counting and ordering.

Some authors begin the natural numbers with 0, corresponding to the non-negative integers 0, 1, 2, 3, and others start with 1, corresponding to the positive integers 1, 2, 3, etc...

Properties of the natural numbers, such as divisibility and the distribution of prime numbers, are studied in number theory. Problems concerning counting and ordering, such as partitioning and enumerations, are studied in combinatorics.

#### Odd Numbers to Twenty

In mathematics, parity is the property of an integer of whether it is even or odd.
An integer's parity is even if it is divisible by two with no remainders left, and its parity is odd if it isn't; that is, its remainder is 1.

#### Prime Numbers

A prime number (or a prime) is a natural number greater than 1 that has no positive divisors other than 1 and itself.

A natural number greater than 1 that is not a prime number is called a composite number. For example, 5 is prime because 1 and 5 are its only positive integer factors, whereas 6 is composite because it has the divisors 2 and 3 in addition to 1 and 6.