47 (number)

← 46 47 48 →
← 40 41 42 43 44 45 46 47 48 49 → List of numbers Integers ← 0 10 20 30 40 50 60 70 80 90 →
Cardinal	forty-seven
Ordinal	47th (forty-seventh)
Factorization	prime
Prime	15th
Divisors	1, 47
Greek numeral	ΜΖ´
Roman numeral	XLVII
Binary	1011112
Ternary	12023
Senary	1156
Octal	578
Duodecimal	3B12
Hexadecimal	2F16

47 (forty-seven) is the natural number following 46 and preceding 48. It is a prime number.

Mathematics

• There are forty-seven integers that cannot be written as a sum of distinct numbers whose reciprocals sum to 1. The largest such number is 77.

• 47 is the largest member of the simplest Cunningham chain, {2, 5, 11, 23, 47} where each prime member is one more than twice the previous. 47 is equivalently the fifth Thabit number of the form $3\cdot 2^{n}-1$ for positive $n$, relating to the pair of amicable numbers {17296, 18416}.

In science

47 is the atomic number of silver.

Astronomy

The 47-year cycle of Mars: after 47 years – 22 synodic periods of 780 days each – Mars returns to the same position among the stars and is in the same relationship to the Earth and Sun. The ancient Mesopotamians discovered this cycle.

In popular culture

Pomona College

The Smith Clock Tower (right) at Pomona College has been set up to chime on the 47th minute of the hour.

The number 47 has historical implications to Pomona College, a liberal arts college in Claremont, California, and has been incorporated into various aspects of campus life. The tradition began in the summer of 1964, when two students, Laurie Mets and Bruce Elgin, conducted a research project seeking to find out whether the number occurs more often in nature than would be expected by chance. They documented various 47 sightings, and professor Donald Bentley produced a false mathematical proof that 47 was equal to all other integers. The number became a meme among the class, which spread once the academic year began and snowballed over time.

Notable 47 sightings include the fact that Pomona is located off of exit 47 of Interstate 10, and the fact that the largest residential building on campus, Mudd-Blaisdell (formally Florence Carrier Blaisdell and Della Mullock Mudd Hall, a title with 47 characters), was completed in 1947 and contains a staircase with 47 balusters.

Many Pomona alumni have deliberately inserted 47 references into their work. Joe Menosky (class of 1979), a writer for Star Trek: The Next Generation, inserted 47 mentions into nearly every episode of the show, a practice that has been picked up by other Star Trek writers. Pomona hosts a community service–oriented celebration every April 7 (abbreviated 4/7 in the U.S.). In the early 2010s, the college's clock tower was set up to chime on the 47th minute of the hour.

Other

Late rapper Capital Steez was infatuated with the number 47 and what it meant spiritually. He believed the number 47 was the "perfect expression of balance in the world", representing the tension between the heart and the brain (the fourth and sixth chakra, respectively.) The number featured on the cover of AmeriKKKan Korruption, stylized to resemble a swastika.