21 (twenty-one) is the natural number following 20 and preceding 22.

Twenty-one is the fifth discrete Semiprime and the second in the (3.q) family. With 22 it forms the second discrete Semiprime pair. As it is a semiprime with both its prime factors being Gaussian primes, 21 is then a Blum integer.

Twenty-one is a Fibonacci number, a Harshad number, a Motzkin number, a triangular number and an octagonal number, as well as a composite number, its proper divisors being 1, 3 and 7.

21 is the sum of the first six natural numbers (1+2+3+4+5+6=21).

21 has an aliquot sum of 11 though it is the second composite number found in the 11-aliquot tree with the abundant square prime 18 being the first such member. Twenty-one is the first number to be the aliquot sum of three numbers 18, 51, 91.

21 appears in the Padovan sequence, preceded by the terms 9, 12, 16 (it is the sum of the first two of these).

The sum of divisors for the integers 1 through 6 is 21.

21 is a repdigit in base 4 (111).

21 is the smallest natural number that is not close to a power of 2, 2^n, where the range of closeness is +- n.

Avram Noam Chomsky (/ˈnoʊm ˈtʃɒmski/; born December 7, 1928) is an American linguist, philosopher,cognitive scientist, historian, and activist. He is an Institute Professor and Professor (Emeritus) in the Department of Linguistics & Philosophy at MIT, where he has worked for over 50 years. Chomsky has been described as the "father of modern linguistics" and a major figure of analytic philosophy. His work has influenced fields such as computer science, mathematics, and psychology.

Ideologically identifying with anarcho-syndicalism and libertarian socialism, Chomsky is known for his critiques of U.S. foreign policy and contemporary capitalism, and he has been described as a prominent cultural figure. His media criticism has included Manufacturing Consent: The Political Economy of the Mass Media (1988), co-written with Edward S. Herman, an analysis articulating the propaganda model theory for examining the media.

According to the Arts and Humanities Citation Index in 1992, Chomsky was cited as a source more often than any other living scholar from 1980 to 1992, and was the eighth most cited source overall. Chomsky is the author of over 100 books. He is credited as the creator or co-creator of the Chomsky hierarchy, the universal grammar theory, and the Chomsky–Schützenberger theorem.

Lionel Andrés Messi (Spanish pronunciation: [ljoˈnel anˈdɾes ˈmesi], born 24 June 1987) is an Argentine footballer who plays in La Liga for FC Barcelona and is the captain of the Argentina national team, mainly as a forward.

By the age of 21, Messi had received Ballon d'Or and FIFA World Player of the Year nominations. The following year in 2009 he won both the Ballon d'Or and FIFA World Player of the Year, and won the inaugural FIFA Ballon d'Or in 2010 and 2011. He also won the 2010–11 UEFA Best Player in Europe Award. Messi is the fourth football player to win three Ballons d'Or, after Johan Cruyff, Michel Platini and Marco van Basten and the second player to win three consecutive Ballons d'Or, after Michel Platini. His playing style and small stature have drawn comparisons to Diego Maradona, who himself declared Messi as his "successor".

Messi has won five La Liga titles and three Champions League titles. In 2012, Messi made UEFA Champions League history by becoming the first player to score five goals in one match, in a 7–1 win against Bayer Leverkusen. With two goals in the next round against A.C. Milan, he matched José Altafini's record of 14 goals in a single UEFA Champions League/European Cup season. He also became the first player to top-score in four successive European Champion Clubs' Cup campaigns. He set the world record for most goals scored in a season during the 2011–12 season, with 73 goals.