20080305, 23:47  #1 
Nov 2007
home
5^{2} Posts 
Modular Subset "Product" Problem
Is there info on how to solve this type of problem somewhere?
Suppose you have a set of congruences A[i] mod B[i] such that A[i] is a random integer and B[i] is a random prime. (the primes are not repeated) Now merge one congruence A[i] mod B[i] with another congruence and join the resulting congruence with other unjoined congruences as many times as needed until you get a congruence with the lowest ratio of A/B that is possible from the set of congruences. 
20080306, 02:35  #2  
Nov 2003
2^{2}·5·373 Posts 
Quote:
GIBBERISH. You have failed to specify the meaning of "merge one congruence with another". Does "unjoined" mean the same as "not yet merged"? One presumes so, but the word "join" is also undefined. Just because "join" and "merge" are synonyms in English does not guaranteee that they must mean the same thing in a math problem. And you have not defined A or B. You have defined A[i] and B[i], but the meanings of A, B, and A/B are vague mysteries. And finally, there is no such thing as a "random integer" or "random prime". They don't exist. 

20080306, 03:39  #3 
Nov 2007
home
5^{2} Posts 
Joining/merging means using the Chinese remainder theorem on congruences (a1 mod b1) and (a2 mod b2) to get another congruence of the form (a3 mod b1*b2).
For a congruence (A mod B) or (3 mod 11) the ratio A/B is 3/11 or 0.272727272727272727..... The set of congruences can be: (2 mod 3);(3 mod 5);(4 mod 7);(1 mod 11) The Chinese remainder theorem can be used on more than two congruences at the same time. This problem involves using Chinese remainder theorem on a specific subset of these congruences to produce a congruence A mod B such that the ratio A/B is smaller than the ratio produced by using the Chinese remainder theorem on any other subset of congruence set. I have no comment for your disbelief in random numbers. 
20080306, 04:06  #4 
Undefined
"The unspeakable one"
Jun 2006
My evil lair
13754_{8} Posts 
I remember a report once stating that 17 is the least random number.
Ask people to think of a random number and 17 is given more often than any other number. 
20080306, 09:14  #5 
Dec 2005
304_{8} Posts 
I thought it was 37

20080306, 09:27  #6 
"Lucan"
Dec 2006
England
14512_{8} Posts 
OK. And what is the smallest uninteresting number?

20080306, 09:58  #7 
Dec 2005
2^{2}·7^{2} Posts 
well, 43 obviously
which makes it very uninteresting indeed 
20080306, 10:02  #8 
"Nancy"
Aug 2002
Alexandria
2,467 Posts 
Reminds me of a code snipped I've seen somewhere:
Code:
/* Returns a random integer */ int random() { return 4; /* Chosen by fair dice roll */ } 
20080306, 12:01  #9  
Nov 2003
2^{2}×5×373 Posts 
Quote:
numbers. I said that random integers and random primes do not exist. They do not. Random integers DRAWN FROM A SUBSET OF Z with a specified density function exist. There is no way to draw an integer at random from the set of all integers. There is no way to draw a prime at random from the set of all primes. You must specify a density function. Although I do not have the details, I am sure that your problem is NPComplete. I have a tentative sketch, based on taking logarithms after combining two of your congruences that reduces your problem to a subsetsum problem. 

20080306, 18:36  #10  
Bamboozled!
"πΊππ·π·π"
May 2003
Down not across
2^{3}×11^{3} Posts 
Quote:
My density function is that the probability of drawing x from Z is 1/N if 1<=x<=N and 0 otherwise. Paul 

20080307, 16:14  #11  
Nov 2007
home
31_{8} Posts 
Quote:
If this problem involved multiplying a bunch of numbers together then it might work. I called this a product problem because multiplication is the closes analogy I know to operation of Chinese remainder theorem. The best algorithm I have so far involves finding "common remainders of remainders." For example: (9 mod 33) and (2 mod 7) makes (9 mod 231) because (9 mod 7) = (2 mod 7) Now if there was a way to use Chinese remainder theorem on a "small congruence" and a "large congruence which has a small remainder" such that the "large congruence's remainder increases but not by too much" this algorithm be usable in practice because the first algorithm would be able to be reused efficiently. 

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