„DA(x, y) = acos(sim(x,y))
„DA(x, y) = acos(x.y/|x||y|))
DA(x, x) = 0
DA(x, y) = DA(y, x)
DA(x, y) + DA(y, z) ³ DA(x, z)
DA(0, 0) = 0 and DA(x, 0) = p/2 by
definition
DA(ax, by) = DA(x, y) with ab > 0
DA(ax, by) = p - DA(x, y) with ab < 0
DA(x+x, x+y) = DA(x, x+y) £ DA(x, y)