Twentyone

Twentyone refers to the natural number 21, the integer that follows twenty and precedes twenty-two. It is commonly written as twenty-one, though some stylized forms use twentyone without a hyphen. In decimal notation it is 21; in binary it is 10101, in hexadecimal 15, and in Roman numerals XXI.

As a mathematical object, twentyone is a semiprime, the product of the primes 3 and 7. It has divisors 1, 3, 7, and 21. It is the third odd composite number after 9 and 15. It is the sixth triangular number, equal to 1+2+3+4+5+6. It is also a Fibonacci number (F8) and a Harshad number in base 10, since 2+1 = 3 and 21 is divisible by 3. Its proper divisors sum to 11, making it a deficient number; it is not a perfect square or cube.

Culturally, twentyone carries various significance. In the United States, 21 is the legal drinking age and marks a threshold for certain legal rights. In games such as blackjack, the objective is to reach a total of 21. The number appears in diverse contexts, including road numbers, sports statistics, and literary or popular references, reflecting its recurring role as a milestone in numerically oriented systems.