For any sequence a ̲ over Z / ( 2 2 ) , there is an unique 2-adic expansion a ̲ = a ̲ 0 + a ̲ 1 · 2 , where a ̲ 0 and a ̲ 1 are sequences over { 0 , 1 } and can be regarded as sequences over the binary field GF ( 2 ) naturally. We call a ̲ 0 and a ̲ 1 the level sequences of a ̲ . Let f ( x ) be a primitive polynomial of degree n over Z / ( 2 2 ) , and a ̲ be a primitive sequence generated by f ( x ) . In this paper, we discuss how many bits of a ̲ 1 can determine uniquely the original primitive sequence a ̲ . This issue is equivalent with one to estimate the whole nonlinear complexity, NL ( f ( x ) , 2 2 ) , of all level sequences of f ( x ) . We prove that 4 n is a tight upper bound of NL ( f ( x ) , 2 2 ) if f ( x ) ( mod 2 ) is a primitive trinomial over GF ( 2 ) . Moreover, the experimental result shows that NL ( f ( x ) , 2 2 ) varies around 4 n if f ( x ) ( mod 2 ) is a primitive polynomial over GF ( 2 ) . From this result, we can deduce that NL ( f ( x ) , 2 2 ) is much smaller than L ( f ( x ) , 2 2 ) , where L ( f ( x ) , 2 2 ) is the linear complexity of level sequences of f ( x ) .
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