In the hypercomplex numbers we describe units as $e_n$ for example, $e_0 = 1, e_1 = i, e_2 = j, e_3 = k ...$ if x is an hypercomplex we can present as $x = x_0e_0+x_1e_1+x_2e_2+x_3e_3+x_4e_4+...$
Can we extend this units to negative of n for example what is $e_{-1}$? Can we reach thats units properties using general formulas at hypercomplex numbers?