71 (seventy-one) is the natural number following 70 and preceding 72.

Look up seventy-one in Wiktionary, the free dictionary.

In mathematics

71 is:

• the 20th prime number. The next is 73, with which it composes a twin prime.
• a permutable prime and emirp with 17.
• the largest number which occurs as a prime factor of an order of a sporadic simple group.
• the sum of three consecutive primes: 19, 23, and 29.
• a centered heptagonal number.^[1]
• an Eisenstein prime with no imaginary part and real part of the form ${\displaystyle 3n-1}$.
• a Pillai prime, since ${\displaystyle 9!+1}$ is divisible by 71, but 71 is not one more than a multiple of 9.^[2]
• the largest (15th) supersingular prime, which is also a Chen prime.^[3]
• part of the last known pair (71, 7) of Brown numbers, since ${\displaystyle 71^{2}=7!+1}$.
• the twenty-third term of the Euclid–Mullin sequence, as it is the least prime factor of one more than the product of the first twenty-two terms.^[4]
• the smallest positive integer d such that the imaginary quadratic field ${\displaystyle {\text{Q)){\sqrt {-d))}$ has class number = 7.^[5]
• the algebraic degree of Conway's constant, a remarkable number arising in the study of look-and-say sequences.
• the sum of primes less than 71 (2 through 67) is 568, which is divisible by 71.
• 71 is the only number between 1 and 100 that is one less or one more than divisible by all numbers from 2 to 10 (70: 2, 5, 7, 10) (72: 2, 3, 4, 6, 8, 9).
• 71 is an index of a prime Lucas number. ^[6]

In science

• The atomic number of lutetium, a lanthanide

In astronomy

• Messier object M71, a magnitude 8.5 globular cluster in the constellation Sagitta
• The New General Catalogue object NGC 71, a magnitude 13.2 peculiar spiral galaxy in the constellation Andromeda

In other fields

Seventy-one is also:

• The number of different characters that can be used with a standard English keyboard, excluding uppercase letters
• The registry of the U.S. Navy's nuclear aircraft carrier USS Theodore Roosevelt (CVN-71), named after U.S. President Theodore Roosevelt
• The number of the French department Saône-et-Loire
• The year AD 71, 71 BC, or 1971
• In the Mexican television sitcom, El Chavo del Ocho, Doña Clotilde lives in the apartment number 71, being therefore known as "La Bruja del 71" (Spanish: "The Witch from Number 71")
• Lockheed SR-71 Blackbird long-range, Mach 3 strategic reconnaissance aircraft
• 71 Fragments of a Chronology of Chance, a film by Michael Haneke
• 71: Into the Fire, South Korean film about the Korean War
• The form number for the United States Office of Personnel Management for requesting a leave of absence
• The Irish Rail 071 Class diesel locomotive
• In the Jason Bourne novels by American author Robert Ludlum the department within the CIA which made Bourne and the other super agents is called Treadstone Seventy-One.
• SR-71, an American alternative rock band
• The number of the laps of the Austrian Grand Prix, Mexican Grand Prix, and Brazilian Grand Prix