:: deftheorem Def4 defines * XCMPLX_0:def 4 :
for x, y being Complex
for b₃ being number holds
( b₃ = x * y iff ex x1, x2, y1, y2 being Element of REAL st
( x = [*x1,x2*] & y = [*y1,y2*] & b₃ = [*(+ ((* (x1,y1)),(opp (* (x2,y2))))),(+ ((* (x1,y2)),(* (x2,y1))))*] ) );