numbers the anaesthetist

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Today is the thirteenth of July.

Thirteen is a number widely considered unlucky. According to one estimate, triskaidekaphobia - fear of the number 13 - costs America more than $1bn each year in absenteeism and cancellations.

In Christian tradition, the bad luck of 13 is linked to the number present at the Last Supper, though that superstition only dates back to the Middle Ages, and many earlier civilizations, notably Babylonian and Old Norse, had registered a dislike for the number 13 centuries earlier.

Mathematically, oddly enough, 13 is one of the "lucky" numbers, defined as follows:

Write down all the integers: 1 2 3 4 5... and cross out every second one, leaving 1 3 5 7...

The smallest remaining number apart from 1 is 3, so cross out every third number. That leaves 1 3 7 9 13... Now the smallest apart from 1 and 3 is 7, so delete every seventh.

Repeating this process, we arrive at the "lucky" numbers:

1 3 7 9 13 15 21 ... .

Apart from being lucky, 13 is also:

The average age at which an American develops a phobia.

The waist measurement in inches decreed for ladies at the court of Catherine de Medici.

The number of Wembley stadia needed to hold the spoil from the Channel Tunnel.

The percentage of the world's bananas grown in India.

The estimated percentage of Britons on a diet at any given moment.

Competition: Five more of our "26 L of the A" (Letters of the Alphabet) teasers with three Chambers Dictionary prizes for correct disentanglings received by 25 July.

5 T on a F

6 F in a F

81/2 F by F

26 F in a Y

32 D F at W W F

Entries to: Pastimes, the Independent, 1 Canada Square, Canary Wharf, London E14 5DL.

29 June answers and prizes:

1 Man's Meat is Another Man's Poison; 8 Maids a-Milking; 9 Men's Morris (or Mythological Muses); 26 Miles in a Marathon; 1609 Metres in a Mile. Winners: Mrs PRM Jackson, Mrs Claire Whitaker, Paul Livesey.