Number 81

A page of the Numeropedia - the Special Encyclopedia of Numbers

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[Chemistry]  The atomic number of Thallium (Tl).

 

[Chemistry]  The number of stable elements. Elements with atomic number 43 (Technetium, Tc), 61 (Promethium, Pm) and from 84 (Polonium, Po) up are radioactive.

 

[Game]  The number of squares on a Shogi (Japanese chess) board (9´9).

 

[Game]  Sudoku is played on a 9´9 grid, which is divided into nine 3´3 boxes. The object is to fill it so that every row, column and 3´3 box contains each of 9 digits 1-9 once.

 

[Flag]  The flag of the British Commonwealth (of 54 independent countries) displays a yellow globe at the center on a blue background, encircling by 81 yellow lines forming letter C.

 

[Library]  ISBN-81 and ISBN-93 are the International Standard Book Number codes for books published in India.

 

[Math.]  81 = 92 = 82+42+12 = (8+1)2. The smallest square number that is the sum of 3 different square numbers. The largest 2-digit square number.

 

[Math.]  81 = (1+2+2+4)2 = 13+23+23+43.

 

[Math.]  81 = (1+2)4 = [(15+25)+(17+27)]/2.

 

[Math.]  81 = 9×9 and, reversely, 18 = 9+9.

 

[Math.]

987,654,321×09 = 08,888,888,889

987,654,321×18 = 17,777,777,778

987,654,321×27 = 26,666,666,667

987,654,321×36 = 35,555,555,556

987,654,321×45 = 44,444,444,445

987,654,321×54 = 53,333,333,334

987,654,321×63 = 62,222,222,223

987,654,321×72 = 71,111,111,112

987,654,321×81 = 80,000,000,001.

 

[Math.]  81 = 92 = (8+1)2

813 = 531,441  = (5+31+44+1)3

814 = 43,046,721  = (4+3+0+46+7+21)4

815 = 3,486,784,401  = (3+4+8+6+7+8+44+01)5

816 = 282,429,536,481  = (2+8+2+4+2+9+5+36+4+8+1)6

819 = 150,094,635,296,999,121 = (1+5+0+0+9+4+6+3+5+2+9+6+9+9+9+1+2+1)9.

 

[Math.]  1/81 = 0.012345679012345679… or 0.012345679. Multiples of 012345679 by 1, 10, 19, 28, 37, 46, 55, 64 and 73 yield all 9 rotations of the digits of the number itself: 012345679, 123456790, 234567901, 345679012, 456790123, 567901234, 679012345, 790123456 and 901234567. Moreover, multiples of 012345679 by a number from 1 to 80 of the form x (mod 9), where x = 1, 2, 4, 5, 7 or 8, will yield a 9-digit number, including all but one digit from 0 to 9. The missing digit is (9–x), again taking only values: 8, 7, 5, 4, 2 or 1. (Adding extra 0 to any 8-digit numbers). Its multiple by 81 is 999,999,999.

 

[Phone]  International telephone dialing code for Japan.

 

[Tech.]  The “number system” code of an EAN-13 barcode for Italy’s products: 80, 81, 82 and 83.

 

[Tech.]  Automobile Tire’s Speed Rating

Tire Rating

L

M

N

P

Q

R

S

Max. Speed (mph)

Max. Speed (kph)

75

120

81

130

87

140

93

150

100

160

106

170

112

180

Tire Rating

T

U

H

V

W

Y

 

Max. Speed (mph)

Max. Speed (kph)

118

190

124

200

130

210

149

240

168

270

186

300