\(\overrightarrow {{u_1}} .\overrightarrow {{u_2}}  = 0\)

\(\overrightarrow {{u_1}} .\overrightarrow {{u_2}}  = \overrightarrow 0 \)       

\(\left[ {\overrightarrow {{u_1}} ,\overrightarrow {{u_2}} } \right] = \overrightarrow 0 \)        

\(\left[ {\overrightarrow {{u_1}} ,\overrightarrow {{u_2}} } \right] = 0\)