. Proof, For some i, the prefix w is of the form w = z 1 . . . z i?1 v i , with v i a prefix of z i . Let ? = (1/?) · (3?) We consider two cases: Case 1: v i is long

K. Then and |. (-v-i-|-x-i?1-y-i?1-)-?-|v-i-|-?-3?-·-n-i-?-|v-i, ? ?)|v i |. This implies K(v i | z 1 . . . z i?1 ) > (1 ? ?) · |v i | ? O(1) ? (1 ? 2?)|v i |, because each z j can be constructed from x j and y j . By induction, it follows that K(z 1 z 2 . . . z i?1 v i ) ? (1 ? 3?)|z 1 z 2 . . . z i?1 v i |. For the induction step, the argument goes as follows: K(z 1 z 2 . . . z i?1 v i ) ? K(z 1 . . . z i?1 ) + K(v i | z 1 . . . z i?1 ) ?O(log(m 1 + . . . + m i?1 ) + log

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