Exponential Expression Divisibility

AN4 is Comparative Divisibility.

This next problem is related to one we’ve done before, but I think it merits its own entry.

The problem: Exponential Expression Divisibility.  This is problem AN5 in the appendix.

The description: Given two sequences A = a1..an and B=b1..bof positive integers, and an integer q. Does \prod_{i=1}^{n} (q^{a_{i}}-1) divide \prod_{i=1}^{m} (q^{b_{i}}-1)?

Let A={2,3} and B={1,4,5}.  If q=4, then the first product comes to 15×63=945 and the second product comes to 3x255x1023 = 782,595.  These two numbers don’t divide.

Reduction: Plaisted’s paper calls this a “refinement” of Polynomial Non-Divisibility.

The reduction for that built a polynomial called “Poly(Cj)”  and showed xN-1 is a factor of \prod_{j=1}^{k}Poly(~Cj)(x) if and only if S is inconsistent.  As I said in that post, there was a lot I don’t understand about it, because he doesn’t give clear proofs or explanations.  He uses some more manipulations to say that the SAT formula is inconsistent iff 2mn-1 divides \prod_{j=1}^{k} \frac{(2^{mn}-1) * Denom(C_j)(2^m)}{Num (C_j)(2*m)} .  (“Num” and “Den” were used in the creation of “Poly”).  This gives us a restatement of the Exponential Expression Divisibility problem where q =2.  Since there was nothing special about the 2, it could work for any integer and thus the reduction works in general.

Difficulty: 9.  Just like the Polynomial Non-Divisibility problem, I have a very hard time following what he’s doing here.  I wish I could find some better explanations

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