_i.e._ We shall have a 4 × 4 series matrix in which all the elements
except those on the diagonal from left up to right down are zero, and
the elements in this diagonal agree with each other, and are each equal
to the above mentioned combination in (36).
The determinant of _f_ is therefore the square of the combination, by
Det^{½}_f_ we shall denote the expression
Det^{½}_f_
= _f₃₂_ _f₁₄_ _f₁₃_ _f₂₄_ + _f₂₁_ _f₃₄_·