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+ The fear of the number 13 is called triskaidekaphobia.

+ 213 - 1 is a Mersenne prime.

+ There are 13 Archimedean solids.

+ The smallest emirp.

+ 132 = 169 and its reversal 312 = 961.

+ 13 is the only prime that can divide two successive integers of the form n2 + 3. [Monzingo]

+ 1313 - 13 + 1 is prime. [Luhn]

+ A "baker's dozen" is a group of 13. Its origin can be traced to a former custom of bakers to add an extra roll as a safeguard against the possibility of twelve weighing light.

+ There are 13 tricks in each hand of bridge. [Goren]

+ 132 divides 12! + 1.

+ The smallest inconsummate number in base 2. [Conway]

+ 13 is half of -13 + 33. [Stark]

+ The olive branch on the back of a U.S. one-dollar bill has 13 leaves. [Weirich]

+ The number (coincidentally prime) of known positive integers which are not the sum of two semiprimes. [Capelle]

+ Eight additional primes can be produced by altering one digit (11, 17, 19, 23, 43, 53, 73, 83).

+ The United States flag once had 13 stars and 13 stripes, which represented the 13 original colonies.

+ The smallest prime whose digits are reversed in base (1 + 3). [Haga]

+ There is no elliptic curve over the rationals Q having a rational point of order 13. [Mazur and Tate]

+ 132 = 7 + 8 + ... + 19. [Bertuello]

+ 132 can be expressed as the sum of 1, 2, 3, 4, 5, 6 distinct squares. [Crespi de Valldaura]

+ The longest recorded flight of a chicken is 13 seconds.

+ 131211109876543212345678910111213 is prime. [Kulsha]

+ 132 turned upside down is prime.

+ ELEVEN + TWO = TWELVE + ONE. Can you solve this doubly-true anagrammatical equation?

+ 13 is the smallest prime which is the sum of two distinct prime squares (22 + 32). [Gevisier]

+ Euclid (circa 300 B.C.) and Diophantus (circa 300 A.D.) each composed 13 books.

+ In May 2001, Ford announced that it would replace 13 million tires on its Ford Explorer SUVs. [Caldwell]

+ Using the first three primes we have: 23 + 5 = 13. [Trotter]

+ Alfred Hitchcock's directorial debut was the film Number 13, which was never completed. [Liebert]

+ The term paraskevidekatriaphobia was coined by therapist Dr. Donald Dossey from the Stress Management Center/Phobia Institute in Asheville, North Carolina, and refers to the fear of Friday the 13th. He claims that when you learn how to pronounce the word (pair-uh-skee-vee-dek-uh-tree-uh-FOH-bee-uh), you'll be cured of the affliction. [Hammond]

+ Osama bin Laden is written with 13 letters. [Gevisier]

+ A Turing Machine with four conditions can write a maximum of 13 characters. [Luhn]

+ The Beatles recorded 13 albums during the time in which they were together as a group. [Litman]

+ The sum of primes up to 13 is equal to the 13th prime. [Gupta]

+ M(13) can be expressed as the sum of 13 consecutive primes (599 + 601 + ... + 661). [Vrba]

+ The sum of the remainders when 13 is divided by all the primes up to 13. [Murthy]

+ The smallest prime whose sum of digits is a square. [Gupta]

+ The Munster's, in the 1960's America television comedy, lived at 1313 Mockingbird Lane.

+ Euler had 13 children, only 5 of whom reached adolescence and 3 of whom survived him. [Poo Sung]

+ The sum of the fourth powers of the first 13 primes plus one is prime (6870733). [Patterson]

+ The 13th of May 2011 will be a "double Friday the 13th," i.e., the sum of the digits of 5/13/2011 equals 13. The next time this will happen in a prime year is 1/13/2141. [Caldwell]

+ The Jewish sage Moses Maimonides established 13 principles of the Jewish faith during the Middle Ages. [Croll]

+ Ironically, 13 is a lucky number. [Emmert]

+ The largest known Fibonacci prime index of a Fibonacci prime. [Rupinski]

+ A concatenation of the first two triangular numbers. [Gupta]

+ The sum of first 13 prime numbers is 238 (whose sum of digits is 13). [Gupta]

+ The smallest prime that can be expressed as the sum of two primes (2 + 11) and two composites (4 + 9) in only one way. [Gupta]

+ There are exactly 13 regular rank four locally projective abstract polytopes. [Hartley]

+ The largest minimal prime in base 3. [Rupinski]

+ 13 is the smallest prime that cannot be a member of a Sophie Germain pair. [King]

+ 13333333333333/13 is prime. The numerator has thirteen 3's following 1. [Poo Sung]

+ 213 - 13 is prime. [Schlesinger]

+ Beethoven's Sonata in C min "Pathetique" is Opus 13. [Necula]

+ The unique prime sum of squares of two consecutive primes (2^2 + 3^2). [Gallardo]

+ The dice game Yahtzee consists of 13 rounds. [Bailey]

+ Three planes can cut a donut into a maximum of 13 parts. [Laurv]

+ 132 = 83 - 73, i.e., one solution of the diophantine equation z2 = x3 - y3. [Jun Mizuki]

+ The U.S. House has voted a total of 13 articles of impeachment against U.S. Presidents: 11 against Andrew Johnson and 2 against Bill Clinton. 13, 11, and 2 are all prime. [Wolfe]

+ A female Virginia opossum usually has 13 nipples: twelve efficiently arranged in an open circle with one in the center. The length of gestation for this curious marsupial is about 13 days. [McCarthy]

+ Apollo Lunar Mission number 13, which was aborted while enroute to the Moon because of an explosion of a fuel cell in the service module, left the launch pad at 13:13 (CST) hours military time and the accident occurred on April 13. [Cooley]

+ The first multidigit half-period prime, i.e., those for which the period equals (p - 1)/2.

+ The 13th Mersenne prime is 2521 - 1 and the 13th Lucas number is 521. [Poo Sung]

+ pi(13) = 1! * 3!. [Gupta]

+ The fear of Friday 13th can be traced back to the suppression of the Templars and the confiscation of their property by Philip IV of France on Friday 13th in October 1307. [Bassano]

+ The first Fibonacci emirp. [Silva]

+ Patau syndrome, also known as trisomy 13, is the consequence of a rare chromosomal abnormality. [Smith]

+ Girolamo Cardano (1501-1576) divided cubic equations into 13 types (excluding x^3 = c and equations reducible to quadratics) in his work Ars Magna, which was the first Latin treatise devoted solely to algebra. [Poo Sung]

+ Write thirteen 13's and delete the last digit. Luckily, you get a prime. [Avrutin]

+ The only known multidigit number n such that sigma(n) is the reversal of prime(n). [Capelle]

+ In five-card poker, the chance of holding two pairs or a better hand is approximately 1/13. [Post]

+ 13 is the smallest prime having only one 1. [Poo Sung]

+ 13 is the hypotenuse of the unique Pythagorean triangle whose area numerically equals its perimeter. [Beedassy]

+ 13 is the smallest prime number p such that (p+2)^2 divides the least common multiple of the squares of the p first numbers. [Rodrigo]

+ There are 13 composite numbers between 13 and its reversal. [Silva]

+ The longest name of a book of the KJV Bible is Thessalonians with 13 letters. [Silva]

+ A three-digit number abc is divisible by 13 if 13 divides a + 4b + 3c.

+ The only prime subscript of a triangular number that is also a square pyramidal number : 1 + 2 + 3 + ... + 11 + 12 + 13 = 12 + 22 + 32 + 42 + 52 + 62. [Beedassy]

+ The only Wilson emirp. [Loungrides]

+ The smallest prime formed from consecutive odd digits. [Silva]

+ In the Walt Disney movie "The Computer Wore Tennis Shoes," Dexter Reilly is asked to raise 13 to the 9th power (and then take the square root of this).

+ The number of brilliant Harshad (or Niven) numbers in base 10. [Capelle]

+ The smallest idoneal emirp. [Loungrides]

+ The smallest emirp Delannoy number. [Loungrides]

+ In 2007, 13-digit International Standard Book Number (ISBN) were introduced. [McCranie]

+ The floor function of e^e - phi^phi, where phi is Golden Ratio. [Firoozbakht]

+ 5*11 - 2*3*7 = 13 (first six primes). [Firoozbakht]

+ 13!+31!-1 is prime. [Silva]

+ The prime whose square is equal to the sum of squares of all digits in which primes end: 132 = 12 + 22 + 32 + 52 + 72 + 92. [Beedassy]

+ Primelephas was an extinct genus of elephants and mammoths that averaged 13 feet in length.

+ At most 13 baseball players can be in play: 9 fielders, 3 runners, and 1 batter. [Poo Sung]

+ There are 13 positive numbers n such that n is equal to the number of 2's in the decimal digits of all natural numbers smaller or equal to n. [Capelle]

+ Counting 0 and 1 as the first two members of the Fibonacci series, term 13 is a perfect square (144). [Fellows]

+ PG-13 is a movie rating that admits all ages but suggests parental guidance of children under age 13. [Teague]

+ The lesser member of the smallest twirp, i.e., a emirp pair such that each member also belongs to a twin prime pair. In this case, the associated primes are 11 and 29. [Green]

+ The smallest prime sum of semiprimes. [Silva]

+ The only Fibonacci prime equal to the sum of the squares of two consecutive Fibonacci primes. [Silva]

+ 13 = (1*3)^0 + (1*3)^1 + (1*3)^2. [Asa]

+ The smallest prime number having exactly one representation as sum of squares greater than one. Note that its reversal is the largest one. [Capelle]

+ There are exactly 13 positive integers that cannot be written as the sum of four hexagonal numbers. Their sum is a prime number whose sum of digits is 13. [Capelle]

+ 13^13 - pi(13)^pi(13) is prime. It is the largest known prime with this property. The next prime of this form (if it exists) will have more than 50000 digits. [Firoozbakht]

+ The smallest prime such that if its digits are rendered in a prime base p, the result, 13 (base p), is never prime. [Green]

+ There are 13 cards of each suit in a deck of playing cards. [Green]

+ The smallest prime that when concatenated to its Collatz sequence (13 --> 40 --> 20 --> 10 --> 5 --> 16 --> 8 --> 4-2-1) inexorable ending 4-2-1 forms another prime: 13421. Note that a reverse concatenation also yields a prime (12413). [Beedassy]

+ Metatron's Cube is created by joining the centers of 13 circles arranged in a "Flower of Life" pattern.

+ If we count all the traditional constellations, there are 13 in the zodiac. [Graham]

+ "COSMOS" by Carl Sagan was originally a 13-part television series.

+ The smallest prime that is equidistant by a prime (3) between the sum of squares of its digits (12 + 32) and the square of the sum of its digits (1 + 3)2. [Beedassy]

+ 'Friday the 13th' is a franchise that consists of several movies, a television show, novels, comic books, and related merchandise. Submitted on Friday the 13th. [Green]

+ The Dynasty of Primes was formed by the first 13 Cybertronians. [Jetfire]

+ The smallest prime whose index is a perfect number. [Pol]

+ The product of 13 and seven is a perfect number in Abbott and Costello's math spoof (from "In The Navy," 1941).

+ 13 can be partitioned in 101 ways. Note that pi(101) + pi(13) - 1 equals the reversal of 13. [Jevtic]

+ 13 squared equals the sum of the first five emirps. [Silva]

+ Oprah Winfrey ran away from home at the age of 13 and was sent to a juvenile detention home, only to be denied admission because all the beds were filled.

+ The number of five-digit primes formed from the five nonzero nonprime digits. [Silva]

+ 13+10(1+2+3+4+5+6+7+8+9+10+11) is prime for each and every summation from 1 to its twin prime pair 11. [Haddish]

+ In the great ancient Indian epic Mahabharata, the righteous Pandava clan, after losing their staked kingdom through deceit in a game of dice schemed out by the envious Kaurauva cousins, were forced to undergo 13 years of exile. [Beedassy]

+ Mice have only 13 pairs of ribs. [Post]

+ Religious scholar A. C. Bhaktivedanta Swami Prabhupada gives the equivalent length of a yojana as approximately 13 kilometers. [Beedassy]

+ "Remarkably, it turns out that for N = 13 the set of operators of 'exceptional' type (4) is empty, i.e., any Hamiltonian operator of order N = 13 can be taken by a contact transformation to an operator of type (1), (2) or (3)! We conjecture that the same holds for all N > 13." Reference: On Classification of Poisson Vertex Algebras by Alberto De Sole, Victor Kac, and Minoru Wakimoto. [Post]

+ pi(pi(1) + pi(2) + ... + pi(13)) = sigma(13). It is the only number with this property. [Firoozbakht]

+ A coven usually consists of 13 witches. [Cheek]


Can you find a total of 13
triangles in this figure?
13 Triangles

+ 13 is the smallest prime p such that p^2 and (p+1)^2 are anagrams (next such prime is 157). [Seidov]

+ 13 describes its neigboring prime gaps, before and after. There is 1 composite before it and 3 composites after it. [Honaker]

+ There are 13 basic points of belief to which Mormons subscribe.

+ Currently (between the years 1901 and 2099) the Julian calendar is 13 days behind the Gregorian calendar. [Gupta]

+ A path on a chessboard from c5 to d4 in 13 straight queen moves that cover all squares is impossible.

(There are 55 curios for this number that have not yet been approved by an editor.)

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