XXIV. On the tangential of a cubic
In my “Memoir on Curves of the Third Order, ”I had occasion to consider a derivative which may be termed the "tangential” of a cubic, viz. the tangent at the point ( x , y , z ) of the cubic curve (*)( x , y , z ) 3 = 0 meets the curve in a point ( ξ , n , ζ ) which is the tangential of the first-mentioned point; and I showed that when the cubic is represented in the canonical form x 3 + y 3 + z 3 +6 lxyz = 0 , the coordinates of the tangential may be taken to be x ( y 3 — z 3 ) : y ( z 3 — x 3 ) : z ( x 3 — y 3 ). The method given for obtaining the tangential may be applied to the general form ( a , b , c , f , i , j , k , l )( x , y , z ) 3 : it seems desirable, in reference to the theory of cubic forms, to give the expression of the tangential for the general form; and this is what I propose to do, merely indicating the steps of the calculation, which was performed for me by Mr. Creedy. The cubic form is ( a , b , c , f , g , h , i , j , k , l )( x , y , z ) 3 , which means ax 3 + by 3 + cz 3 +3 fy 2 z +3 gz 2 x +3 hx 2 y +3 iyz 2 +3 jzx 2 +3 hxy 2 +6 lxyz ; and the expression for ξ is obtained from the equation x 2 ξ =( b , f , i , c )( j , f , c , i , g , l )( x , y , z ) 2 ,—( h , b , i , f , l , k )( x , y , z ) 2 ) 3 — ( a , b , c , f , g , h , i , j , k , l )( x , y , z ) 3 (C x +D), where the second line is in fact equal to zero, on account of the first factor, which vanishes. And C, D, denote respectively quadric and cubic functions of ( y , z ), which are to be determined so as to make the right-hand side divisible by x 2 the resulting value of ξ may be modified by the adjunction of the evanescent term