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From Wikipedia, the free encyclopedia

221 (two hundred [and] twenty-one) is the natural number following 220 and preceding 222.

In mathematics

221 is:

  • a product of two prime numbers, 13 and 17, making it a semiprime
  • the sum of five consecutive prime numbers (37 + 41 + 43 + 47 + 53)
  • the sum of nine consecutive prime numbers (11 + 13 + 17 + 19 + 23 + 29 + 31 + 37 + 41)
  • a centered square number[1]
  • Mertens function(221) = 5 a record high that stands until 554
  • If you deal 2 cards from a single deck of 52 cards, you have a 1 in 221 chance of getting 2 aces.

In other fields

221 is:

Historical years


  1. ^ "Sloane's A001844 : Centered square numbers". The On-Line Encyclopedia of Integer Sequences. OEIS Foundation. Retrieved 2016-05-26.
  2. ^ Conan Doyle, Sir Arthur A Study in Scarlet pub. Wordsworth Editions Limited (2004) p.10 "We met next day, as [Holmes] had arranged, and inspected the rooms at No. 221B Baker Street, of which he had spoken at our meeting." ISBN 978-1-84022-411-5
This page was last edited on 7 December 2018, at 19:26
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