21 (number)

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

In mathematics

21 is:

  • the fifth discrete semiprime and the second in the (3 × q) family. With 22 it forms the second discrete Semiprime pair.
  • a Blum integer, since it is a semiprime with both its prime factors being Gaussian primes.
  • a Fibonacci number.
  • a Harshad number.
  • a Motzkin number.
  • a triangular number.
  • an octagonal number.
  • a composite number, its proper divisors being 1, 3 and 7.
  • the sum of the first six natural numbers (1 + 2 + 3 + 4 + 5 + 6 = 21), making it a triangular number.
  • the sum of the sum of the divisors of the first 5 positive integers.
  • the smallest non-trivial example of a Fibonacci number whose digits are Fibonacci numbers and whose digit sum is also a Fibonacci number.
  • a repdigit in base 4 (1114).
  • the smallest natural number that is not close to a power of 2, 2n, where the range of closeness is ±n.
  • the smallest number of differently sized squares needed to square the square.
  • 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.

    XXI (band)

    XXI (formerly, A Feast for Kings (AFFK)) is an American Christian hardcore, metal, and rock band, and they play a version of post-hardcore, progressive metal and progressive rock styles of hardcore punk, metalcore, and rock, with some more melodic hardcore and melodic metalcore elements. They come from Carrollton, Kentucky, where they were formed in 2010, with lead vocalist Eric Gentry. Eric's death in 2014 was the reasoning behind the band's name change, with 'XXI' (Roman numerals for '21') being Eric's age at the time of his death. Their first extended play, Hell on Earth, was released in 2014, shortly before Eric's death, and under their former name. The debut studio album, Inside Out, was released with Carson Butcher as lead vocalist on Tooth & Nail Records in 2015. This album was their breakthrough album on both the Billboard magazine Christian Albums and Heatseekers Albums charts.

    Background

    The band is from Carrollton, Kentucky, where they formed in 2010 under the name A Feast for Kings, with their members being lead vocalist, Eric Gentry; background vocalist and guitarist, Seth Weigand; guitarist, Jamie King; bassist, Robbie Barnett; and drummer Carson Butcher. After Gentry's death, the group changed their name to XXI to memorialize his age at his time of death. Their drummer went on to become their lead vocalist, and the bassist took over on the drums.

    Podcasts:

    PLAYLIST TIME:

    Latest News for: xxi

    Fitch Afirma las Calificaciones de las Siefores de Afore XXI Banorte (Fitch México SA de CV)

    Public Technologies 08 Apr 2025
    ). Evento Relevante de Calificadoras. FECHA. 08/04/2025. BOLSA MEXICANA DE VALORES, S.A.B ... Fitch Afirma las Calificaciones de las Siefores de Afore XXI Banorte.

    Annual Report for Fiscal Year Ending December 31, 2024 (Form 10-K) (AEI Income & Growth Fund XXI LP)

    Public Technologies 28 Mar 2025
    AEI INCOME & GROWTH FUND XXI LIMITED PARTNERSHIP ... The registrant is comprised of AEI Fund Management XXI, Inc ... AEI INCOME & GROWTH FUND XXI LIMITED PARTNERSHIP ... AEI Income & Growth Fund XXI Limited Partnership ... AEI Fund Management XXI, Inc.
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