If L is NP-complete then how can I prove that $L_1$:
$L_1= (1$ { $0, 1$ }$^∗) \cup ${ $0x | x \in L$}
is also NP- complete.
My thoughts: A reduction from (for example) SAT to L can be converted to a reduction to { $0x | x \in L$} by adding a 0 to the output.