Differential operator, four-space

In dealing with fields that vary over time and space, we will need various differential operators. In the nonrelativistic theory of electrodynamics the gradient operator, V, and the time derivative, d/dr, are used. From our experience in the previous chapter with mixing of space and time coordinates under Lorentz transformations, we might expect these to combine in a four-space differential operator also. Indeed, in our notation. 

Mathematical operations have specific rules for the use of mathematical symbols with SI units. A space or a half-high dot represents the multiplication of units a negative exponent, horizontal line, or slash represents the division of units, and if these mathematical symbols appear in the same line, parentheses must differentiate them. The percent sign (%) denotes the number 0.01 or 1/100, so that 1%= 0.01, 30% = 0.30, and so forth. Arabic numerals with the appropriate SI or recognized unit indicate the values of quantities. Commas are not used to separate numbers into groups of three. If more than four digits appear on either side of the decimal point, a space Table 3. Prefixes. separates the groups of three. 

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