Eleven things you never knew about the number 11

Here, in recognition of today’s special date and in time for your elevenses, are eleven things you may have overlooked about the number eleven.

1. Today’s date, 11.11.11, is one of only 12 this century, where day, month and year are written the same – and where it means the same on both sides of the Atlantic.

2. Eleven is the first palindromic number… the date 11.11.11 is triply so and the only date of its kind this century.

3. The volume control on the BBC’s online catch-up service, iPlayer, goes up to 11.

4. Eleven is the smallest double digit prime number – and the fourth Sophie Germain prime, the third safe prime, the fourth Lucas prime, the first repunit prime, and the second good prime. Displayed on a calculator, 11 is a strobogrammatic prime and a dihedral prime because it reads the same whether the calculator is turned upside down or reflected on a mirror, or both.

5. There’s a really easy way to see if a number is divisible by 11:
Take every digit of the number located in odd position and add them up, then take the remaining digits and add them up. If the difference between the two sums is a multiple of 11, including 0, then the number is divisible by 11. For instance, if the number is 65,637 then (6 + 6 + 7) – (5 + 3) = 19 – 8 = 11, so 65,637 is divisible by eleven. This technique also works with groups of digits rather than individual digits, so long as the number of digits in each group is odd, although not all groups have to have the same number of digits. For instance, if one uses three digits in each group, one gets from 65,637 the calculation (065) – 637 = -572, which is divisible by eleven.
Got that?

6. The number 11 bus is a low-cost way of sightseeing in London and featured in the film Da Vinci Code.

7. Eleven is very big in Canada. Due to Canada’s federal nature, eleven legally distinct Crowns exist in the country, with the Monarch being represented separately in each of 10 provinces as well as at the federal level. The maple leaf on the Flag of Canada has eleven points. The Canadian one-dollar coin is a hendecagon, an eleven-sided polygon. Clocks depicted on Canadian currency, for example the Canadian fifty-dollar bill, show 11:00.

8. Sunspot activity has been strongly linked to the quality of French wines, especially from Bordeaux.The periodicity of a sunspot cycle is about 11 years.

9. There are eleven space time dimensions in the unifying version of string theory called M-Theory.

10. No one knows why soccer, hockey, cricket and American football teams all have eleven players (OK, I made that up, but no one I know, knows. I am open to ideas…).

And finally,

11. Today, 11 November, is Remembrance Day (aka Armistice Day or Poppy Day) in the UK and Commonwealth and Veterans Day in the USA. It recalls the official end of World War in 1918 “at the 11th hour of the 11th day of the 11th month”.

Hope you enjoyed that. Now, just over a year to plan 12.12.12…

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