55 (number)

55 (fifty-five) is the natural number following 54 and preceding 56.

Mathematics
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 $$55 = 5 \times 11$$ are the respectively the third and fifth, where the first two Fermat primes of the form $$2^{2^n} + 1$$ are $$3$$ and $$5$$ (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 $$7 - 2 = 5$$, 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).

Science

 * 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

Music

 * 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.

Transportation

 * In the United States, the National Maximum Speed Law prohibited speed limits higher than 55 mph from 1974 to 1987

Film

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

Years

 * AD 55
 * 55 BC
 * 1755
 * 1855
 * 1955

Other uses

 * 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.