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April 19th, 2009, 01:23 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, 03:06 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 fourdigit sequence is 1/10^4.

April 20th, 2009, 09:43 AM  #3  
Newbie Joined: Mar 2009 Posts: 13 Thanks: 0  Re: Checking Phone Numbers Uisng Set Theory Quote:
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, 11:37 AM  #4  
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 Quote:
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.  

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