why is the number 13 considered unlucky?
819146
As you know a year is a full rotation of earth around the sun and initially month was the full rotation of moon around earth. Sumerians created their first calendar using this information. The problem with this is that a full rotation of moon is 29.5 days then 12 months make just 354 days. This is shorter than a year. Therefore, a synchronization was needed. In order to be able to synchronize their calendar people added another 13th month in every three years. However a standardization could not be achieved, different people added the 13th month at different times. This created a lot of problems especially for trade and contracts. As a result of this experience people thought that something is bad about 13 and they changed their calendar in such a way that months are no longer equal to the full rotation of moon around earth. There are still some calendars with a month equal to the full rotation of moon. For example, "honey moon" is a saying from those old days, because it was the tradition that father of the bride provided a fermented honey drink to the groom for a full month after the marriage.
324897
Could be 10,14,13. The odd terms differ by 3 (eg. 4-1,7-4), the even terms differ by 4.
942916
your question has a problem...............\nif the sum is 13, then the other number has to be -35, which will not meet your other condition
513542
They have a 1 in 32 chance
89160
It is purely coincidence.\n\nCoincidence has always given people thought.\n\n“It is no great wonder if in the long process of time, while fortune takes her course hither and thither, numerous coincidences should spontaneously occur.  If the number and variety of subjects to be wrought upon be infinite, it is all the more easy for fortune, with such an abundance of material, to effect this similarity of results” – Sertorius Page 678.
497428
It seems that a large proportion of people have answered your first point (and very well I might add). But regarding the '20 consecutive tails' part, strictly speaking the answer is that the chances of the coin coming down 'heads' is still 50-50.  However, the chances of a coin coming down 'tails' on 20 consecutive occasions is so remote, that we would have to consider the possibility that the coin has been tampered with!
778238
I am a teacher of mathematics and in a 31 year career I have had it happen to me exactly 1 time!!! It is a very very low probability
945700
Statically, if you try luck for many time, once it will be successfull.\n\nTRY MANY  TIME, ONCE MAY BY GOD's mistaken grace it will be.
2233
Triskaidekaphobia is the fear of 13, a number commonly associated with bad luck in Western culture. While fear of the number 13 can be traced back to medieval times, the word triskaidekaphobia itself is of recent vintage, having been first coined by Coriat (1911; Simpson and Weiner 1992). It seems to have first appeared in the general media in a Nov. 8, 1953 New York Times article covering discussions of a United Nations committee. \n\nThis superstition leads some people to fear or avoid anything involving the number 13. In particular, this leads to interesting practices such as the numbering of floors as 1, 2, ..., 11, 12, 14, 15, ... (Sloane's A011760; the "elevator sequence"), omitting the number 13, in many high-rise American hotels, the numbering of streets avoiding 13th Avenue, and so on. \n\nApparently, 13 hasn't always been considered unlucky. In fact, it appears that in ancient times, 13 was either considered in a positive light or (more commonly) not at all (Adams). The association of bad luck with the number 13 has been attributed to the fact there were 13 people at the Last Supper of Jesus, although this association seems to have originated only in medieval times. \n\nThe association has also been linked to that fact that lunisolar calendars (such as the Hebrew and Chinese calendars) must have 13 months in some years in order to synchronize the solar and lunar cycles, while the solar Gregorian calendar  in universal current usage always has 12 months in a year. \n\nTriskaidekaphobia also may be related to Norse mythology, which tells how the god Odin invited eleven of his closest friends to a dinner party at his home in Valhalla, only to have his party crashed by Loki, the god of evil and turmoil, thus giving a total of 13 people. The legend goes on to related how Balder, one of the most beloved gods, tired to throw Loki out of the party, resulting in a scuffle and ultimately Balder's death with a mistletoe-tipped arrow. \n\nFear of the number 13 also leads to fear of Friday the thirteenth  (a fear recently dubbed paraskevidekatriaphobia), despite the fact that Friday turns out to be the most common weekday on which the 13th of a month can occur in the Gregorian calendar . The association of bad luck with Friday appeared in Chaucer's Canterbury Tales in the late 14th century ("and on a Friday fell all this misfortune"), but references to Friday as a day associated with ill luck in general first appear around the middle of the 17th century (Mikkelson and Mikkelson). In particular, it appeared in numerous publications as a particularly unlucky day to start a new venture (beginning a journey, giving birth, getting married, moving, starting a new job, etc.) beginning around 1800 (Mikkelson and Mikkelson). \n\nWhile 13 is associated with the number of people at the Last Supper, the Crucifixion took place on a Friday, thus leading to an association of back luck with the combination of this number and day. Note that the association of Friday the thirteenth with the arrest of Jacques de Molay, Grand Master of the Knights Templar, on Friday, October 13, 1307 by King Philip IV of France is a modern-day invention (Mikkelson and Mikkelson). \n\nParaskevidekatriaphobia is probably the most widespread superstition in the United States, possibly affecting tens of millions of Americans. Interestingly, a study by Scanlon et al. (1993) published in the prestigious British Medical Journal which analyzed the relation between health, behavior, and superstition surrounding Friday 13th in the United Kingdom by comparing the ratio of traffic volume to traffic accidents on Friday the 6th and Friday the 13th over a period of years found that, "The risk of hospital admission as a result of a transport accident may be increased by as much as 52%. Staying at home is recommended." \n\nWhile there does not appear to be hard evidence to support the claim, it has been suggested that $800 to $900 millio
391186
13!/(13-4)! = 17160 if order matters\n\n13!/(4!*(13-4)!) = 715 if order does not matter\n\nI give up after this one ...\n\nNow that I think of it, if you're allowed to reuse numbers, you would have\n\n13^4 = 28561
1018404
The number is too small for a real "pattern" to emerge.\nThe digits could be the first 10 values of some sequence given by some ( many ) recurrence relation, could be ...\n\nIn order for a proper pattern to emerge, it should be much larger...
832564
The answer is one chance in 2^5  or 1 in 32.\n\nNotice that the chances are the same as flipping five heads in a row.
959868
I don't think it's bad luck.  Check this out....\n\nhttp://en.wikipedia.org/wiki/Triskaidekaphobia\n\nPlus in old times witches covens were said to have 13 members.
75027
Why Are We Afraid of Friday the 13th?\n\nIn the Bible the numbers 7, 12 and 40 have symbolic meaning and appear throughout the Old and New Testaments. The number 12 is considered a lucky number. Consequently, the number, which follows 12, was thought to be evil. The number 13, in the Christian faith, is the number of parties at the Last Supper, with the 13th guest at the table being the traitor, Judas. When Christians combine this day and number, the combination can only hold special significance.\n\nKnown scientifically as "Tridecaphobia," fear of the number 13 is probably the most common of all superstitions. Buildings avoid numbering the 13th floor. Airplanes avoid the 13th aisle. And almost every North American knows that Friday the 13th is considered a bad luck day.\n\nAs we have stated in other parts of this site... if you believe it, it will be... so if you believe that Friday the 13th is unlucky, it will be... on the other hand, for me, Friday the 13th has always brought good luck!\n\nFor most folks, Friday the thirteenth is considered the unluckiest of days, unless you were born on Friday the 13. If you were born on this day then Friday the 13 is your lucky day. \n\nThe origins of Friday superstitions are many. One of the best known is that Eve tempted Adam with the apple on a Friday. Tradition also has it that the Flood in the Bible, the confusion at the Tower of Babel, and the death of Jesus Christ all took place on Friday.\n\nThe modern basis for the Friday the 13th superstition stems from Friday October the 13th, 1307. On this date, the Pope of the church in Rome in Conjunction with the King of France, carried out a secret death warrant against "the Knights Templar". The Templars were terminated as heretics, never again to hold the power that they had held for so long. There Grand Master, Jacques DeMolay, was arrested and before he was killed, was tortured and crucified. A Black Friday indeed!\n\nLong before the Bible was written, Friday was considered an important day. Primitive people set aside Fridays as a special time to worship their deities and ask them for good crops, health and happiness. Those who worked on this day were told not to expect "good luck" from the gods. (See our superstitions on the Days of the Week)\n\nThe day Friday was named after Frigg (or Frigga), the Norse goddess of marriage. Later she was confused with the goddess of love, Freya, who in turn became identified with Friday. When the Norsemen and Germanic tribes became Christians, Freya was supposed to have been banished to the mountains as a witch. \n\nFriday came to be called "witches' Sabbath." For it was believed that on this day, each week, twelve witches and the Devil met - thirteen evil spirits up to no good! This is one of the reasons for today's superstition about Friday the 13th.\n\nThe Scandinavian belief that the number 13 signified bad luck sprang from their mythological 12 demigods, who were joined by a 13th demigod, Loki, an evil cruel one, who brought upon humans great misfortune.
518594
I researched this question for you, I discovered more than I expected too. I hope this helps:\n\n\nOrigins\nThe origin of the Friday the 13th superstition has been linked to the belief that there were 13 people at The Last Supper of Jesus, who was crucified on Good Friday, but no evidence has been found that Friday 13th was considered especially unlucky until the 19th century. The number 13, however, has a long history of association with ill-luck. It has been linked to the fact that a lunisolar calendar must have 13 months in some years, while the solar Gregorian calendar and lunar Islamic calendar always have 12 months in a year. Another suggestion is that the belief originated in a Norse myth about twelve gods having a feast in Valhalla. The mischievous Loki gate-crashed the party as an uninvited 13th guest and arranged for Hod, the blind god of darkness, to throw a branch of mistletoe at Balder, the god of joy and gladness. Balder was killed instantly and the Earth was plunged into darkness and mourning as a result. Both the first and last explanations, however, seem more relevant to the superstition linked to having 13 people at the same table during a meal. There is also another theory that Friday the 13th of October 1307 was the day that Philip IV of France arrested hundreds of the French Knights Templar to get their money for the French treasury. One other note which predates all of the aforementioned is that the first Passover seems to have occurred on Friday the 13th. The death of the firstborns of Egypt occurred on a Shabbat on the 14th of Nisan in the evening. But the Jewish calendar counts days from sunset to sunset so this would have been Friday the 13th in terms of the gentile reckoning of the days. (Exodus 12:6) Feminists have argued that because of the lunar year and Friday being named after a goddess in most European pagan calendars, the fear of Friday the 13th is a patriarchal invention, associating femininity with bad luck.\nhttp://www.useless-knowledge.com/1234/ma...\n\nSo what is some of the facts behind these superstitions? Well, first of all, let’s distinguish between two fears: \n\n1) Tridecaphobia – fear of the number 13\n\n2) Paraskevidekatriaphobia – fear of Friday the Thirteenth \n\nThe first one, Tridecaphobia, stems from people who just plain hate the number 13. Period. They will go to all extents to avoid the number. I know a few of these people, and sometimes it gets to be quite annoying. One person in particular that I know is unbelievably superstitious. He won’t write a check out if it has the number 13 on it. He purposely avoids being thirteenth in anything. 13 does not exist for him. 12 then 14 does, but not 13. It’s not part of his vocabulary. Extreme, yes, but he has a phobia, after all. \n\nParaskevidekatriaphobia—don’t even ask me to pronounce it, but it’s quite real. People who have this fear will not go out of their house on that day (Friday the 13th), avoid sharp objects on that day—just avoid it all together. The whole day just doesn’t exist on their calendar.
493543
Friday the 13th is an unlucky day in much of Western Europe, North America, and Australia. Many people avoid travel and avoid signing contracts on Friday the 13th. Floors in tall buildings often skip from 12 to 14. And while the superstition is believed to be fading, it nonetheless has deep roots in both Christian and pagan culture.\nDuring the Middle Ages, the superstition against Friday the 13th grew. On Friday, October 13, 1307, King Philip IV of France ordered the arrests of Jaques de Molay, Grand Master of the Knights Templars and sixty of his senior knights in Paris. Thousands of others were arrested elsewhere in the country. After employing torture techniques to compel the Templars to "confess" to wrongdoing, most were eventually executed and sympathizers of the Templars condemned Friday the 13th as an evil day. Over time a large body of literature and folk wisdom have reinforced the belief. In the 18th century, the HMS Friday was launched on Friday the 13th(shipwrecked)It was never heard from again. Since then, ships are not usually launched on that date.
70456
There are many stories behind the origins of Friday the 13th. The one known to most people is that Eve temped Adam with the apple on a Friday. Another well known story is the death of Jesus Christ took place on Friday. What many don't know is that Friday the 13th had a bad name before the Bible.\n\n\nFriday got its name during Norse Mythology. The day was named after the Norse goddess of marriage Frigga. Frigga is not the one responsible for the badness of the day. Frigga was confused with the Norse goddess Freya who was bannished to the mountains as a witch. It was said that Freya along with 11 other witches and the devil had meetings every week on Friday. It was then when Friday became witches sabbath and the day of bad luck.\n\nFear of Friday the 13th\n\nNumber 13 has long been considered by the superstitious to be unlucky. Many towns and suburbs don’t have 13 as a street number, or 13th Street, and most hotels don't have rooms with 13 on the door. Many tall buildings skip the 13th storey, with the elevator going straight from Floor 12 to 14. \n\nBut when a Frday falls on the thirteenth of the month the superstitions increase. Why is Frday 13 considered unlucky ? There are all sorts of explanations in our modern times, from the alleged inability of ancient people to count past twelve to witches covens of 13, biblical allusions and the massacre of the Knights Templar \n\nFor some reason there is a theory that early people counted only their fingers and feet and so any number greater than 12 was beyond their reckoning. The number 13 would then be mysterious and magical. This can be dismissed as a patronising view of our ancestors who could certainly count a lot higher than twelve.\n\nThe association of Friday the 13th with being unlucky is popularly linked with the Knights Templar, and the date of Friday, October the 13th, 1307 CE. And that's another story in itself\n\nIn pre-Christian cultures Friday was often a day of worship. It was considered lucky in Northern Europe as a day to get married because of its traditional association with love and fertility. Our name for Friday comes from the old Norse deity worshipped on the sixth day, Frigg/Freya who corresponds to Venus, the goddess of love of the Romans. The Latin for the sixth day of the week is "dies Veneris", the day of Venus.\n\nAll that changed when Christianity came along. The goddess Freya with her sacred animal, the cat, was recast in post-pagan folklore as a witch, and her day became associated with evil doings. \n\nHowever, despite all of the reasons and derivations for the unlucky Friday 13, no one has been able to document the existence of such beliefs prior to the 19th century. If people who lived before that perceived Friday the 13th as a day of special misfortune, there's no evidence remains to prove it. Perhaps the idea of a bad luck day is a thoroughly modern phenomenon. \n\nLike most superstitions the real origin of Friday the 13th is subject to great speculation, Personally I chose to treat all superstitions with a great deal of salt, thrown over the left shoulder of course