# 221 (number)

 ← 220 221 222 →
Cardinal two hundred twenty-one
Ordinal 221st
(two hundred and twenty-first)
Factorization 13 × 17
Roman numeral CCXXI
Binary 110111012
Ternary 220123
Quaternary 31314
Quinary 13415
Senary 10056
Octal 3358
Duodecimal 16512
Vigesimal B120
Base 36 6536

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.

221 is:

## Historical years

221 A.D. and 221 B.C.

## References

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