= Solution
\b[<Small data global regularity for wave maps> also holds in three spatial dimensions.] For localized compatible data small in high weighted <Sobolev norms>, the key is the <classical null condition for wave equations>. Each derivative contraction is a <null form for wave equations>, vanishing for parallel null derivatives. The <vector field method for wave equations> exploits derivatives tangent to the <light cone> and weighted <energy estimates> to close the <bootstrap argument>; ordinary $(1+t)^{-1}$ decay alone is insufficient.
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