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DUDE! It's Friday the 13th!

I was late night running just while ago and didn't even remember it was Friday the 13th. I felt the whole time like some1 was watching me from the forest next to the street I run...

Only 18 min left here. Anything can still happen... :rolleyes:
 
A little off track but...
dejo said:
I now consider this one of the luckiest days of my life! The house they are building west of me is ranch-style, therefore preserving (for the most part) a fairly significant portion of my view of the Colorado Front Range. I am so, so happy! :D

The pictures don't really do it justice but give you an idea, at least. Imagine if the new house was a two-storey like the one next door (to the left). I would not have been happy.
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Man I love Colorado....those pics just took me back for a minute. Got engaged in March '04..married March '05.

Something special about those beautiful moutains.:)
 
Measured over many years, the 13th of the month is more often on a Friday than on any of the other six days of the week.

(Just thought I should spoil the fun mood by making it mathematical.)
 
Doctor Q said:
Measured over many years, the 13th of the month is more often on a Friday than on any of the other six days of the week.
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How is that possible...?

Not that I doubt your statistical powers, but do you have some charts to back up that claim...? ;)
 
Mitthrawnuruodo said:
How is that possible...?

Not that I doubt your statistical powers, but do you have some charts to back up that claim...? ;)
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mad jew said:
Measured over infinite years, it should be even, right?
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It's just chance that Friday was the winning day for the 13ths.

First, proof that one of the days was destined to beat the others:
Our (western) calendar is the Gregorian calendar, which has a leap day every 4th year, except every 100th year, except every 400th year.

So consider any range of 400 years. It'll have 300 regular (also called "common") years and 100 leap years, except the years that are multiples of 100 won't be leap years, except for the one that's also a multiple of 400, which will be a leap year after all. The result is 300+4-1 = 303 regular years and 100-4+1 = 97 leap years.

That's 303 x 365 + 97 x 366 = 146,097 days. This is a multiple of 7, meaning that each day occurs exactly 20,871 times per 400-year-cycle. So whatever pattern of weeks we find during any period of 400 years will be repeated for all other 400-year cycles and we can consider just one cycle.

During any range of 400 years, we have 400 x 12 = 4800 months. Some will start on Monday, some on Tuesday, etc. But since 4800 is not divisible by 7, it can't be that the 1st of the month is on a Monday exactly 1/7th of the time, on a Tuesday exactly 1/7th of the time, etc. In other words, the 1st of the month falls on some days of the week more than other days of the week.​
Now, the chance part:
By luck, Sunday is the winner for most common 1st day of the month. The actual numbers are
684 Mondays, 687 Tuesdays, 685 Wednesdays, 685 Thursdays, 687 Fridays, 684 Saturdays, 688 Sundays​

So the 1st of the month occurs more often on a Sunday than on other days. When the 1st is a Sunday (as it is this month), the 13th is on a Friday. And there you have it!

The 13th is on a Friday a whopping 688 / 4800 = 14.3333...% of the time while the 13th is on runners-up Sunday and Wednesday a mere 687 / 4800 = 14.3125% of the time each. And having the 13th on a Thursday is extremely rare, only 684 / 4800 = 14.25% of the time!

And yes I'm glad you asked. :D
 
Thanks Doctor Q, I think you just reassured us of your god status. :eek:

I didn't get the step to Sunday being the most common month starter though. Is that based on a theory or someone actually counting the month starters over a set 400 years?
 
Doctor Q said:
And yes I'm glad you asked. :D
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Thanks... keep 'em coming... :)

Doctor Q said:
The 13th is on a Sunday a whopping 688 / 4800 = 14.3333...% of the time while the 13th is on runners-up Sunday and Wednesday a mere 687 / 4800 = 14.3125% of the time each. And having the 13th on a Thursday is extremely rare, only 684 / 4800 = 14.25% of the time!
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Friday, prehaps... ;)
 
Mitthrawnuruodo said:
Friday, prehaps... ;)
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Whoops. All that work and I typed "Friday" wrong in the conclusion. I fixed it above so people will get a proper explanation. Thanks for spotting that.

Yes, mad jew, you just count 'em to see how many of each you get. Since others have already done this, I didn't bother to check their work.
 
Doctor Q said:
Yes, mad jew, you just count 'em to see how many of each you get. Since others have already done this, I didn't bother to check their work.
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Yeah, I thought so. I was just kinda hoping there'd be some Mega Impressive Equation™ that'd do it for us. :p
 
Doctor Q said:
Whoops. All that work and I typed "Friday" wrong in the conclusion. I fixed it above so people will get a proper explanation. Thanks for spotting that.
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That's what I'm here for... :)

I actually read it as "Friday" in the first read-through... just caught it on the follow-up browse-through... ;)
 
I had to read through that about three times to follow the logic....I get it mostly, but damn I need a beer and I just read it! Very interesting.
 
Hope your on a mac, as they keep you secure from Friday the 13th's. Only windows does random crashes :) :D :D :p


2nyRiggz said:
dude dont jinx it....my computer is fine my computer is fine my computer is fine:cool:


Bless
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