20071203, 16:29  #1 
Dec 2007
2 Posts 
Solving linear systems modulo n
i need to solve a linear system modulo a composite number.
if this number is n = p*q where p and q are prime, i know i have to solve the system modulo p and modulo q and combine the solutions with the chinese remainder theorem. but what if this number is a power of a prime (n = p^k) or a composition of powers (n = p1^k1 * p2^k2) ? How should i proceed? 
20071203, 18:03  #2  
Tribal Bullet
Oct 2004
3·1,163 Posts 
Quote:


20080207, 16:20  #3 
Dec 2007
2_{10} Posts 
Hi,
unfortunately, after all this time, I didn't solve my problem yet: implementing in C the Index Calculus method for computing discrete logarithms. On the web I didn't find anything different by the proposed version of Studholm but it's too diffilcult to me. Does anyone knows a simple implementation of this method? In particular I'm in trouble to code the linear algebra step of the algorithm: how employ Henseltype methods for matrix algebra modulo prime powers and chinese remainder methods for gluing powers of different primes. Any suggestions or link to other resources? Thank you for your reply jasonp! 
20080208, 15:06  #4  
Nov 2003
2^{6}·113 Posts 
Quote:
mathod used by Kevin McCurley etal. in their efforts. If N is the modulus, factor it. Solve the system mod p for each prime p dividing N. You can do this either by the Lanczos or Weidemann method. [or even by Gaussian elim if the matrix is small enough]. Then for primes p that divide N to a higher power than 1, raise the solution to p^k via Hensel's Lemma. [Hensel's lemma says that if A is a solution to f(x) = 0 mod p^k , then a solution mod p^(k+1) will be of the form A + hp for some h.] Now paste everything together by the CRT. For an effective implementation of the CRT, see my joint paper with Peter: P. Montgomery & R. Silverman An FFT Extension to the P1 Factoring Algorithm, in Math.Comp. 

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