Abstract: Using the theories of the minimum error and the k-error linear complexity profile of sequences,m-tight error linear complexity is presented to study the stability of the linear complexity of sequences.First,the structure of the Wei-Xiao-Chen algorithm for the linear complexity of sequences with period 2pn over GF(q) is optimized,where p and q are odd primes and q is a primitive root (mod p2).Second,the union cost is used,so that an efficient algorithm for computing the k-error linear complexity of a sequence with period 2pn  over GF(q) is derived,where p and q are odd primes and q is a primitive root (mod p2).Finally,an efficient algorithm for computing m-tight error linear complexity of sequences with period 2pn over GF(q) is given,where p and q are odd primes and q is a primitive root (mod p2).

Key words: stream cipher, sequence, linear complexity, k-error linear complexity, m-tight error linear complexity