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+ There are 7 letters in TUESDAY, which is the only day of the week whose name contains a prime number of letters. [Gupta]

+ The first prime number of the form 6n + 1. All primes greater than three have either the form 6n + 1 or 6n - 1.

+ Graph theorists have been able to prove that 7 colors are required on a donut-shaped map (i.e., an ordinary one-holed torus) to ensure that no adjacent areas are the same.

+ The first 7 digits of 89 form a prime. [Kulsha]

+ The only single digit prime number to yield a square when its cube and the cube's proper divisors are added. Thus 73 + (1 + 7 + 72) = 202.

+ 7! - 6! + 5! - 4! + 3! - 2! + 1! is prime. [Guy]

+ Seven is the only odd prime that becomes "even" by deleting a letter. [Beedassy]

+ There are only 7 smoothly undulating nine-digit palindromic primes.

+ "Seventh heaven" is the farthest of the concentric spheres containing the stars in the Muslim and Cabalist systems.

+ Pill bugs ("roly-polies") have 7 pairs of legs.

+ In 1992, Bayer and Diaconis showed that after 7 random riffle shuffles of a deck of 52 cards, every configuration is nearly equally likely.

+ The Seven Bridges of Königsberg is a famous solved mathematics problem inspired by an actual place (now Kaliningrad, Russia) and situation. [Millington]

+ The smallest odd full-period prime.

+ If you turn a U.S. one-dollar bill upside down and place your thumb over the eagle's head, the shield becomes the Menorah, or the 7 candlesticks of Israel.

+ Jesus uses the word seven a "perfect number" of times in the last verse of the first chapter in the Book of Revelation. This makes perfect sense because seven is the number of perfection in the Bible. [Zirkle]

+ The only odd prime that solves 6y2 = (x + 1)(x2 - x + 6). [Ljunggren]

+ The first prime happy number. Replace a number by the sum of the squares of its digits, and repeat the process until the number equals 1, or it loops endlessly in a cycle which does not include 1. Those numbers for which this process ends in 1 are called happy numbers. There are exactly 7 prime happy numbers less than 100. Can you find all 7?

+ There are 7 indeterminate forms involving 0, 1, and infinity.

+ 7 Up (or Seven Up) is a brand of a lemon-lime flavored soft drink.

+ 17 + 47 + 47 + 57 + 97 + 97 + 27 + 97 = 14459929. [Randle]

+ Thabit ibn Qurra (circa 850 A.D.) noted that if n>1 and each of p = 3*2^(n-1) - 1, q = 3*2^n - 1, and r =(p+1)*(q+1) - 1 is prime, then (2^n*p*q) and (2^n*r) are amicable pairs. Descartes wrote to Mersenne in 1638 with the pair 9363584 and 9437056 (n = 7).

+ Add 7 to 720(10049 - 1)/99 to get a 99-digit undulating prime. [Pickover]

+ The largest known number of consecutive primes that can be partitioned into two sets such that the difference of their product is unity: (5 * 11 * 13) - (2 * 3 * 7 * 17) = 1.

+ The Clay Mathematics Institute announced in May 2000 a prize of $1,000,000 for each of the 7 unsolved Millennium Prize Problems. The Poincaré conjecture (essentially the first deep conjecture ever made in topology) has been the only Millennium problem solved thus far.

+ If asked to quickly pick a number between 12 and 5, most people will choose 7.

+ If n is composite, the first 7 primes of the form sigma(n)/d(n) are Mersenne prime exponents.

+ Michael Bell proved that 10^(100*n) + n is prime, where n = 7. As of May 2000, it is the only known prime of this form.

+ 7! + 1919 is a prime number with 19's on both ends. [Kulsha]

+ There are 7 different notes in a standard major scale in music as well as in a standard minor scale. [Obeidin]

+ All primes p greater than or equal to 7 divide a concatenation of (p-1) ones. [Murthy]

+ Using A = 1, B = 2, ..., Z = 26, the sum of all the letter values in the word names of the numbers from 1 to 7 is a prime (367). If 0 is included, the sum is yet another prime (431). [Trotter]

+ Double 7! to get the exact number of minutes in a week (7 days). [Luhn]

+ The 'Tractatus Logico-Philosophicus' of Ludwig Wittgenstein brings 7 groups of aphorisms. The 7th group brings only one (a very curious one): Wovon man nicht sprechen kann, darüber muss man schweigen. About what one cannot speak, one must keep silent. [Gevisier]

+ A Mersenne prime (2p - 1) with exponent p = 3. The key prime factor of the perfect number 28. There are twenty-eight 3-digit numbers with digits summing to 7 (e.g., 601; 412) among which exactly 7 (e.g., 241; 313) are prime and also among which one (511) is of the form 2q - 1 where q = 3 * 3. The seven 3-digit primes sum to 2281, a prime with an embedded "28" and with first and last digits summing to 3. Note that the number 2281 is also the exponent of the 17th Mersenne prime.

+ The reversal of (7 - 1)! + 1 is 27 - 1 (a Mersenne prime). [Luhn]

+ There are "7 deadly sins" used in early Christian teachings to educate and protect followers from basic human instincts (pride, envy, gluttony, lust, anger, avarice, and sloth). [Croll]

+ The first number of every sextuplet ends with 7. [Luhn]

+ The sum of the digits of each of the first two multidigit squares is 7. [Sladcik]

+ The smallest prime of all integers n such that Mordell's equation y^2 = x^3 + n has no integral solutions. [Post]

+ There were 7 rings of power for the Dwarf-lords in the stories of J.R.R. Tolkien. [Oldenbeuving]

+ The number of points and lines on the minimal finite projective (or Fano) plane. [Poo Sung]

+ It is possible to place 7 cigarettes in such a way that each touches the others if the length divided by the diameter of each is greater than or equal to 7*sqrt(3)/2. [Gardner]

+ De Morgan suspected Shanks's calculation of pi up to 707 digits was wrong because of the lack of 7s contrary to other digits. [Poo Sung]

+ (77 + 777)/(7 + 7) = 61 is prime. Note that seven 7's are used and 6 + 1 = 7. [Poo Sung]

+ There were 7 papers in the first edition of the Ramanujan Journal. [Croll]

+ There are 7 seals in the New Testament's Book of Revelation. [Croll]

+ There is a persistent rumor that the philosopher G.W.F. Hegel (1770-1831) provided a logical proof within his doctoral dissertation that there could only be 7 planets in the solar system. [Poo Sung]

+ The only prime whose sum of divisors (p+1) is a cube. [Gupta]

+ The smallest prime which can be expressed as the sum of a prime and a composite number (7 = 3 + 4). [Murthy]

+ In most Hindu marriages the bride follows the groom 7 times around the holy fire, which is called a Saptapadi. [Das]

+ The smallest example of x-y,xx-y,xxx-y all being prime is with digits x=7 and y=4. So 7-4=3, 77-4=73, 777-4=773 are all primes. It is the only solution because one of three consecutive numbers x..x-y is divisible by 3. [Luhn]

+ (2^7-1)/(2-1) and (2^7+1)/(2+1) are both primes. [Dobb]

+ Uranus' moon Miranda (as viewed from space) has a "7" embedded in the middle of a rectangular corona that appears to have been formed by viscous icy lavas. Uranus is the 7th planet from the Sun.

+ There are 7 primes less than 77 that have at least one digit being 7 (7, 17, 37, 47, 67, 71, 73). [Bush]

+ 7 is the smallest integer that is not the difference of two primes. [North]

+ The very first Menorah for the Tabernacle as commanded by God to be built in the book of Exodus had 7 lamps in it. [Obeidin]

+ The only prime such that all the positive values of 7-2^k are all primes, where k is not equal to 0. [Russo]

+

Seven Segment Display
Early electronic instruments incorporated alphanumeric displays that employed discrete circuit components known as 7-segment light-emitting diodes (LED's). [McAlee]

+ The smallest Woodall prime. [Russo]

+ The smallest cuban prime. The name has nothing to do with Cuba the country.

+ The bluntnose sevengill shark has 7 long gill slits in front of each pectoral fin. [Jolly]

+ 7 = sod(sod(sod(4444^4444))), where sod(n) is the sum of digits of integer n. [Daly]

+ "Seven Minutes of Terror" is the phrase given to Curiosity's final minutes to landing on Mars. Touchdown on the floor of Gale Crater is expected at 1:17 a.m. (EDT) on 6 August 2012.

+ The Japanese word "Subaru" refers to an open star cluster called the Seven Sisters (Pleiades).

+ Kwanzaa (or Kwaanza) is a 7-day festival (December 26 to January 1) celebrated primarily in the United States, honoring African-American heritage. [Karenga]

+ The number of stars that form the asterism known as the Big Dipper in North America.

+ Roma was built over 7 hills. [Franco]

+ The only prime that can be the digital root (sum of digits computed recursively until one digit remains) of a perfect square. [Rupinski]

+ 13 + 17 + 19 = 7^2. [La Haye]

+ Andrew Wiles spent 7 years working on his solution of Fermat's Last Theorem. [Croll]

+ 7 is the only prime defined as a cube minus one. [Gallardo]

+ In "Western" music's 12-tone scale, ratios of fractions between 2 and 1 with a one-digit denominator that involve 7 are not used. This is true whether 7 is either in the numerator or denominator. Ratios such as 7/4, 7/5, 7/6, 8/7, 9/7, etc. are often common notes in the music of other cultures or blues music, but not the "Western" equal temperament scale. [Obeidin]

+ The maximum number of eclipses of the Sun and Moon that can occur in any one year. [Byrd]

+ 7 is the number of frieze groups, i.e., the groups consisting of symmetries of the plane whose group of translations is isomorphic to the group of integers.

+ In 1995, Ramaré showed that all integers greater than one are the sum of at most 7 prime numbers.

+ The number of distinct prime knots containing 7 crossings. [Tait]

+ 72 can be written as a sum of squares in 72 + 72 ways. [Hartley]

+ The smallest sexy prime triplet that is not part of a sexy prime quadruplet or quintuplet starts at 7. I.e, 7+6 and 7+6+6 are also prime. [Opao]

+ 7^12 reversed is prime. [Patterson]

+ The numbers on opposite sides of a standard die always add up to 7.

+ The number of rank and good ears of corn that came up upon one stalk in Pharaoh's second dream (Genesis 41:5).

+ The sum of seven consecutive primes beginning with 7 is seven times the seventh prime. [Post]

+ The number of visible colors formed by light across a prism. [Gallardo]

+ According to the History Channel - 7 machine tools started the modern age: Lathe, Planer, Mill, Grinder, Drill, Band Saw, and Punch-Press. [McAlee]

+ The first exotic differentiable structure was a 7-dimensional hypersphere. It was found by Milnor (1956) using the calculus of octonions. [Poo Sung]

+ 1!*2!*3!*4!*5!*6!*7! + 1 are twin primes. [Gupta]

+ Atop the Statue of Liberty there are seven spikes, representing the seven seas. [Patterson]

+ 7 of the major planets in our solar system have natural satellites. [Wolfe]

+ 7 is the smallest prime number that is equal to the product minus the sum of twin primes, i.e., 3*5 - (3+5) = 7. [Teofilatto]

+ The smallest prime p such that all five numbers p*10^k+1, k < 6 are primes. [Firoozbakht]

+ 7 is the smallest Carol prime. [McAlee]

+ 7 = 2^5 - 5^2. [Astle]

+ 7 is the only number n such that n = pi(n)! - prime(n). [Firoozbakht]

+ Levy's conjecture (1963) states that all odd numbers greater than or equal to 7 are the sum of a prime plus twice a prime. It was named after Hyman Levy, who was apparently unaware that the conjecture was first stated by Émile Lemoine in 1894. [Capelle]

+ 7 is a double Mersenne prime. [Post]

+ 7 is the first centered hexagonal prime. [Post]

+ The smallest prime followed by a prime number of consecutive composite numbers. [Silva]

+ 7 is the smallest number that cannot be represented as a sum of 3 squares. [Poo Sung]

+ Assuming Goldbach's conjecture, there is no positive number that can be written as the sum of exactly 7 primes in exactly 7 ways. [Hartley]

+ Have you ever noticed that the word "indivisibilities" contains 7 i's?

+ The smallest prime whose reciprocal has semiprime period length. [Post]

+ Whilst every Fibonacci prime number has odd prime subscript, 7 is the first prime not appearing in the Fibonacci sequence. [Beedassy]

+ John Elway, one of the most formidable figures in NFL history, wore number 7 for the Denver Broncos.

+ The smallest prime NSW number. [Beedassy]

+ There are exactly 7 possible basic shapes for six-sided polyhedra (convex hexahedra). [Caldwell]

+ 7 is the only prime in which a/b + c/d + e/f = a/b * c/d * e/f, where the variables a through f represent any nonzero single-digit numbers. [Morse]

+ The smallest hyperfibonacci prime that is not a Fibonacci prime. [Post]

+ Ladybugs have 7 spots on their back. [McCranie]

+ Divisibility test for 7 (or 13): Combine the digits in order into groups of 3 (starting from the right) by alternating them with positive and negative signs. If the result is divisible by 7 (or 13 respectively), then so is the original number. For example, 7 divides 62540982 because 7 divides +(62)-(540)+(982).

+ The smallest prime p that yields a number of the form pp when multiplied by the next prime. [Capelle]

+ Insert 7 in the middle of the 7th power of 7 and you will get a 7-digit prime. [Silva]

+ 7*6*5*4*3*2*1 + 7654321 is a 7-digit prime with first digit 7 and 1*2*3*4*5*6*7 + 1234567 is a 7-digit prime with last digit 7. [Silva]

+ The only prime p such that pi(p2) < (pi(p))2. [Capelle]

+ Patterson's Conjecture: If Pn is the nth prime, then the series (P1-1 + P2-2 + P3-3 + ... + Pn-n) is divisible by Pn if and only if Pn = 7.

+ It is possible to construct an angle of pi/7 using 7 matchsticks. [Beedassy]

+ Although baseball's first World Series, in 1903, was an 8 game series, as was 1912, 1919, and 1921, it has ever since been a best-of-seven-game series. [Post]

+ The dimensions of irreducible modules of the exceptional Lie group G_2 are all congruent to {-1,0,+1} mod 7. A similar phenomenon occurs for all exceptional Lie groups and for some classical ones as well. [Rupinski]

+ The smallest prime subscript n such that pn+1 is the first prime dividing #pn+1, (pn being the nth prime ). [Beedassy]

+ The only prime equal to the difference of the product and the sum of the two previous primes. [Silva]

+ The normal human head bears 7 sense organ openings: 2 eye sockets, 2 ear canals, 2 nostrils, and a mouth. [Beedassy]

+ The only prime that is member of two pairs of cousin primes. [Silva]

+ The 7 heavenly virtues are prudence, temperance, justice, fortitude, charity, hope and faith. [Croll]

+ The Hamming-7 code is a perfect error-correcting code of word-length 7, with 4 message carrying bits and 3 error-correcting bits. [Sirag]

+ The game Tetris uses 7 one-sided tetrominoes (referred to as I, J, L, O, S, T, and Z). [Poo Sung]

+ The largest known prime p such that p! + 1 is the square of a prime. [Beedassy]

+ 7 = 98532/14076 is the unique pandigital fraction expression including 0. [Poo Sung]

+ The only odd prime p whose n-th power pn, for any nonzero n below p, has exactly n digits. [Beedassy]

+ The checksum of the checksum of the first 7 decimal places of the common logarithm of 7 is 7 (log 7 = 0.8450980, 8+4+5+0+9+8+0=34, = 3+4). [Vastvision]

+ There are 72 natural numbers n such that n is equal to the number of 7's in the decimal digits of all numbers smaller or equal to n. [Capelle]

+ The smallest prime whose square is a concatenation of consecutive prime squares. [Silva]

+ The largest prime number whose third power (343) can be displayed on a 12-hour digital clock. [Fellows]

+ 'Oh Thank Heaven for 7-Eleven'. The 7-Eleven (R) name reflects the stores' extended hours - 7 a.m. until 11 p.m., seven days a week. July 11 (7/11) is the official birthday of the 7-Eleven convenience store chain. [Green]

+ The smallest prime with a composite number of partitions. [Pol]

+ The Pollock octahedral numbers conjecture (1850) states that every number can be written as the sum of at most seven octahedral numbers. Frederick Pollock, better known as a lawyer and politician, was a contributor of papers on mathematics to the Royal Society. [Capelle]

+ The number of distinct four of fewer face-joined unit cubes forming the polycube pieces of the 3-by-3-by-3 Soma cube dissection puzzle invented by Piet Hein in 1936. [Beedassy]

+ The largest known prime number that is not the sum of a triangular number, a square and a cube, all of them greater than or equal to 1. [Capelle]

+ Seven prime numbers cannot be written as a sum of squares of primes. Note that 7 is one of them and divides their sum. [Capelle]

+ The largest known prime number p for which the decimal expansion of pp does not contain any pair of consecutive equal digits. [Capelle]

+ There are 7 known natural numbers n such that n is the sum of two consecutive primes but not the sum of two sums of two consecutive primes. [Capelle]

+ There are 7 improper zeroless pandigital fractions that equal 7. For example, 37926/5418 = 7. [Bolling]

+ All multidigit Fermat numbers end in 7. [Gupta]

+ Dubai eGovernment has a logo in line with law number 7 of 2009 issued by H.H. Sheikh Mohammed Bin Rashid Al Maktoum, Vice President and Prime Minister of the UAE and Ruler of Dubai, which is one of the 7 emirates and the most populous state of the United Arab Emirates.

+ The number of capital letters of English alphabet with no symmetry (F, G, J, L, P, Q, R). Note that the letters P, Q and R are often used to represent the primes. [Capelle]

+ The smallest prime p followed by more than one prime prior to 2p. [Silva]

+ The number of strobogrammatic capital letters of the English alphabet (H, I, N, O, S, X, Z). The word strobogrammatic begins with S, the only strobogrammatic letter with a prime index. [Capelle]

+ "The Seven Year Itch" (1955) is a highly-rated comedy film starring Tom Ewell and Marilyn Monroe. The titular phrase refers to a declining interest in a monogamous relationship after seven years of marriage. [Green]

+ All 7 numbers n! + nextprime(n), n < = 7 are primes. [Firoozbakht]

+ Each hind foot of Ichthyostega (one of the first tetrapods) had 7 toes. [Lindsay]

+ A segment called "Seven Prime Questions" first aired on PBS in January 2006.

+ The sum of all untouchable primes. [Loungrides]

+ "The time will come when diligent research over long periods will bring to light things which are now hidden." - Seneca, Natural Questions, Book 7, first century

+ 1! + 2! + 3! + 4! + 5! = 6 (mod 7). It is the largest known prime (up to 10^5) with this property. [Firoozbakht]

+ The only prime, p, such that σ(p3) is a square. [Wesolowski]

+ Every connected (non-empty) one-dimensional topological manifold is homeomorphic to one of these 7: the circle, the closed interval, the open interval (real line), the half-open interval, the closed long ray, the open long ray, or the long line. [Post]

+ The smallest of only two primes less than a titanic prime of the form p^p+p+1, where p is a prime. The other is 31. [Loungrides]

+ 7 is the 3rd lucky number. It is also the rank of the E7 Lie algebra (E6 and E8 are the only other E-type Lie algebras). [Sirag]

+ There are 7 hidden dimensions in 11-d Supergravity, which is the "low energy" approximation to M theory, which also has 7 hidden dimensions. [Sirag]

+ The metric system uses 7 fundamental units of measure: m, kg, K, mol, A, sec, and cd. [Jauregui]

+ The Heawood graph corresponds to the 7-color torus map on fourteen nodes. Note that "Heawood" has 7 letters. [Post]

+ The only two-syllable prime digit in English. [Mia]

+ The only known number whose factorial has 50% of its digits consisting of 0's: 7! = 5040. [Beedassy]

+ Hex-7. [Hein and Nash]

+ The smallest number n that yields two primes when preceded or followed by n ones. [Silva]

+ H-7 is the heaviest isotope of hydrogen observed experimentally in a heavy particle accelerator. [Beedassy]

+ Seven Prime Questions

+ (7, 11, 13) is the first consecutive prime number triple (p, q, r) such that r divides pq+1. Note that the quotient is 6 (the first perfect number). [Loungrides]

+ Scottrade offers $7 online stock trades. If you open a new account, feel free to use my ReferALL code: DRLL0275, and receive 3 free trades! (G. L. Honaker, Jr.)

+ Giraffes have seven neck bones, which is the same as humans and most other mammals but longer. [Capelle]

+ Why is 10 afraid of 7? Because 7 8 9 and 10 is next! [Homewood]

+ The only prime heptagonal number. [Homewood]

+ (123465789 + 987654321) +/-7 are both prime. [Bopardikar]

(There are 50 curios for this number that have not yet been approved by an editor.)

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