### About the Number 13 (Thirteen)

Six days ago it was the lucky day 07/07/07 (which was a Saturday), but today (07/13/2007) is a Friday the 13th. Friday the 13th is considered the unluckiest of days, unless you were born on Friday the 13th. If you were born on this day, then Friday the 13th is your lucky day. In mathworld (number theory), 13 is a Lucky Number.

Every year has at least one Friday the 13th. No year has had (or will have) more than three Friday the 13ths. 2007 had two Friday the 13ths--April and July. 2008 has only one Friday the 13th and that occurs in June.

```   pre-MBNA (MathBabbler Number Analyst)
=====================================
13 is a whole number that's also a positive integer
13 is odd
13 is prime
13 is not a digit
13 in binary is 1101
13 in octal is 15 (and 013 is 11)
13 in hexadecimal is D (and 0x13 is 19)
13 squared is 169
13 cubed is 2197
13! is 6,227,020,800
sqrt(13) = 3.60555128
log(13) = 1.11394335
ln(13) = 2.56494936
______
reciprocal(13) = 0.076923

many buildings with more than 12 floors don't have a floor 13
triskaidekaphobia is the fear of number 13
paraskavedekatriaphobia is the fear of Friday the 13th

MBNA (MathBabbler Number Analyst) output
[inserted on Friday, 6/13/2008]
========================================
13 is happy
13 is not a Harshad number
13 is prime
...Cousin prime with 17
...Sexy prime with 19
...Sexy prime with 7
...Twin prime with 11
...Pythagorean prime
...Star prime
...Left-truncatable prime
...prime triplet {7, 11, 13}
...prime triplet {11, 13, 17}
...prime triplet {13, 17, 19}
...prime quadruplet {5, 7, 11, 13}
...prime quadruplet {11, 13, 17, 19}
13 is undulating
13 in octal is 015
13 in binary is 1101 (is odious)
13 nearest square numbers: -4...3 (9...16 [4])
sqrt(13) = 3.60555
ln(13) = 2.56495
log(13) = 1.11394
```

RoadHacker does not have a Hwy-13 highway sign; however, TempeHiker did find a 13 MPH speed limit sign in a parking lot at the Gateway Center located on 44th Street in Phoenix, Arizona.

Numerous websites, including MathWorld.Wolfram.com, claims "it has been suggested that \$800 to \$900 million are lost each Friday the 13th (Roach 2004) as a result of people avoiding travel, wedding plans, moving, and so on."

This BAB was created during the year 2007. 13 years into the future will be the year 2020.

##### Update::2008.06.13

This number BAB was updated on Friday, 13 June 2008.

```   13 is the sum of the two square-numbers 9 and 4
13 in Roman numeral is XIII
13 in base-13 is 16
```

Creator: Gerald Thurman [gthurman@gmail.com]
Created: 13 July 2007