My Math Forum Checking Phone Numbers Uisng Set Theory

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 April 19th, 2009, 12:23 PM #1 Newbie   Joined: Mar 2009 Posts: 13 Thanks: 0 Checking Phone Numbers Uisng Set Theory Hi everybody, i have a book covering set theory like relations, functions , predicate calculas, bags and how to write them ect. Something that i wanted to know for a while to know from you guys is that if i had a phone number, but wanted to check that the first numbers on a phone number started with, 0798, which were of type number AND no longer than 11 digits long, how could i write this in set theory or what would be the easiest way of doing this? What would i need i.e relations ect cheers
 April 19th, 2009, 02:06 PM #2 Global Moderator     Joined: Nov 2006 From: UTC -5 Posts: 16,046 Thanks: 938 Math Focus: Number theory, computational mathematics, combinatorics, FOM, symbolic logic, TCS, algorithms Re: Checking Phone Numbers Uisng Set Theory If the numbers are all at least four digits long, and each digit is selected (as a phone number) uniformly at random, then the chance of the first number starting with any given four-digit sequence is 1/10^4.
April 20th, 2009, 08:43 AM   #3
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Re: Checking Phone Numbers Uisng Set Theory

Quote:
 Originally Posted by CRGreathouse If the numbers are all at least four digits long, and each digit is selected (as a phone number) uniformly at random, then the chance of the first number starting with any given four-digit sequence is 1/10^4.
I see, but what i want to do is write this out in set theory and use for example sequence operations i.e selection, to check for the first four digits of a phone number rather than an equation. A phone number will be sequence of digits.

In set theory i want to write something along the lines of:

The first 4 numbers at position 0,1,2,3 of a telephone number must be 0798 and less than or equal to 11 digits long. From reading set theory books #s represents the length of a sequence.

i.e
0 -> 0
1 -> 7
2 -> 9
3 -> 8

Hope that makes sense now.

cheers

April 20th, 2009, 10:37 AM   #4
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Math Focus: Number theory, computational mathematics, combinatorics, FOM, symbolic logic, TCS, algorithms
Re: Checking Phone Numbers Uisng Set Theory

Quote:
 Originally Posted by nvidia Hope that makes sense now.
No, actually. It makes no sense to me at all.

I know set theory -- I learned it from Enterton's excellent book -- but I don't have any idea how you want to apply it to your problem.

 Tags checking, numbers, phone, set, theory, uisng

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