A nonzero vector  in 
-dimensional Lorentzian space 
 is said to be positive timelike
 if it has imaginary (Lorentzian) norm and if its first component
 
 is positive. Symbolically, 
 is positive timelike if both
and
hold. Note that equation (6) above expresses the imaginary norm condition by saying, equivalently, that the vector  has a negative squared norm.
 
         
	    
	
    
