9999

9999 is the natural number that follows 9998 and precedes 10000. In decimal notation, it is the largest four-digit number and a four-digit repdigit composed entirely of the digit 9.

In mathematics, 9999 has the prime factorization 3^2 × 11 × 101. Equivalently, 9999 = 9 × 1111 = 99 × 101 = 10^4 − 1. It is a palindromic number in base 10 and is just below a power of ten.

In other bases, 9999 is represented as 10011100001111 in binary and 0x270F in hexadecimal.

In mathematics and computing, 9999 is often used as an example of a four-digit boundary value; in programming and testing, inputs such as 9999 are used to test formatting and overflow handling.

Culturally, in East Asian numerology the digit 9 is associated with permanence and longevity; repeating nines such as 9999 are sometimes regarded as especially auspicious, though beliefs vary by tradition and individual.