 These simple forms "x"2 ± "y""k"+1 are said to give normal forms for the type "Ak"±-singularities.Notice that the "A"2"n"+ are the same as the "A"2"n"− since the diffeomorphic change of coordinate ("x","y") → ("x", −"y") in the source takes "x"2 + "y"2"n"+1 to "x"2 − "y"2"n"+1.So we can drop the ± from "A"2"n"± notation.