221 (number)

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← 220 221 222 →
Cardinal two hundred twenty-one
Ordinal 221st
(two hundred twenty-first)
Factorization 13 × 17
Greek numeral ΣΚΑ´
Roman numeral CCXXI, ccxxi
Binary 110111012
Ternary 220123
Senary 10056
Octal 3358
Duodecimal 16512
Hexadecimal DD16

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

Contents

In mathematics

Its factorization as 13 × 17 makes 221 the product of two consecutive prime numbers, the sixth smallest such product. [1]

221 is a centered square number, [2] an Ulam number, [3] and a brilliant number, [4] meaning that its prime factors have the same amount of digits.

In Other Fields

The year 221 BC marked the end of the Warring States period and the beginning of the Qin dynasty in China. [5]

221b Baker Street is the address of the fictional detective Sherlock Holmes, created by Sir Arthur Conan Doyle. [6]


References

  1. ↑ Sloane, N. J. A. (ed.). "SequenceA006094(Products of 2 successive primes)". The On-Line Encyclopedia of Integer Sequences . OEIS Foundation.
  2. ↑ Sloane, N. J. A. (ed.). "SequenceA001844(Centered square numbers)". The On-Line Encyclopedia of Integer Sequences . OEIS Foundation.
  3. ↑ Sloane, N. J. A. (ed.). "SequenceA002858(Ulam numbers)". The On-Line Encyclopedia of Integer Sequences . OEIS Foundation.
  4. ↑ Sloane, N. J. A. (ed.). "SequenceA078972(Brilliant numbers)". The On-Line Encyclopedia of Integer Sequences . OEIS Foundation.
  5. ↑ "Qin Dynasty". Smithsonian Institution. Retrieved 15 April 2025.
  6. ↑ "221B Baker Street". Sherlock Holmes Museum. Retrieved 2025-04-15.


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