Number 55

55 is the fifteenth discrete semiprime, and the second with 5 as the lowest non-unitary factor. Thus, of the form 5 × q with q a higher prime, in this case equal to 11.

It contains an aliquot sum of 17; the seventh prime number, within an aliquot sequence of one composite number (55, 17, 1, 0) that is rooted in the 17-aliquot tree.

55 is the tenth Fibonacci number. It is the largest Fibonacci number to also be a triangular number (the tenth as well); it is furthermore the fourth doubly triangular number.

55 is also an early member inside other families of polygonal numbers; it is strictly (when including 0 as the zeroth indexed member) the fifth:

It is also the fourth centered nonagonal number, and the third centered icosahedral number.

In decimal, 55 is a Kaprekar number, whose digit sum is also 10. It is the first number to be a sum of more than one pair of numbers which mirror each other (23 + 32 and 14 + 41).

Fermat primes

The prime indices in the prime factorization of ${\displaystyle 55=5\times 11}$  are the respectively the third and fifth, where the first two Fermat primes of the form ${\displaystyle 2^{2^{n}}+1}$  are ${\displaystyle 3}$  and ${\displaystyle 5}$  (11 is also the third super-prime). Where 17 — the aliquot part of 55 — is the third Fermat prime, the fifty-fifth prime number 257 is the fourth such prime number. The base-ten digit representation of the latter satisfies a subtractive concatenation of ${\displaystyle 7-2=5}$ , wherein 77 is the fifty-fifth composite number. In decimal representation, the fifth and largest known Fermat prime is 65537, which contains a "55" string inside (and where as a number, 637 is the eleventh non-trivial decagonal number).

• The atomic number of caesium.

Astronomy

• Messier object M55, a magnitude 7.0 globular cluster in the constellation Sagittarius
• The New General Catalogue object NGC 55, a magnitude 7.9 barred spiral galaxy in the constellation Sculptor

• The name of a song by Kasabian. The song was released as a B side to Club Foot and was recorded live when the band performed at London's Brixton Academy.
• "55", a song by Mac Miller
• "I Can't Drive 55", a song by Sammy Hagar
• "Ol' '55", a song by Tom Waits
• Ol' 55 (band), an Australian rock band.
• Primer 55 an American band
• Station 55, an album released in 2005 by Cristian Vogel
• 55 Cadillac, an album by Andrew W.K.

• In the United States, the National Maximum Speed Law prohibited speed limits higher than 55 miles per hour (90 km/h) from 1974 to 1987

• 55 Days at Peking a film starring Charlton Heston and David Niven

• AD 55
• 55 BC
• 1755
• 1855
• 1955

• Gazeta 55, an Albanian newspaper
• Agitation and Propaganda against the State, also known as Constitution law 55, a law during Communist Albania.
• The code for international direct dial phone calls to Brazil
• A 55-gallon drum for containing oil, etc.
• The Élysée, the official residency of the French Republic president, which address is 55 rue du Faubourg-Saint-Honoré in Paris.

• 55th Regiment of Foot (disambiguation)
• Channel 55 (disambiguation)
• Type 55 (disambiguation)
• Class 55 (disambiguation)
• List of highways numbered 55