Now they could find what really satisfies them and do that.
It has been shown above that a covariant, in general, satisfies four partial differential equations.
If they be willing faithfully to serve it, that satisfies.
Hence in all there are mn such systems. If, therefore, we have a third equation, and we substitute each system of values in it successively and form the product of the mn expressions thus formed, we obtain a function which vanishes if any one system of values, common to the first two equations, also satisfies the third.
Each symbol a is associated with its supplement a which satisfies the equivalences a+a = i, aa = o, the latter of which means that a and a have no region in common.
