An important case is formula_25. The interest here is in how large formula_26 must be for an embedding, in terms of the dimension formula_27 of formula_18. The Whitney embedding theorem states that formula_29 is enough, and is the best possible linear bound. For example, the real projective space RP"m" of dimension formula_27, where formula_27 is a power of two, requires formula_29 for an embedding. However, this does not apply to immersions; for instance, RP2 can be immersed in formula_33 as is explicitly shown by Boy's surface—which has self-intersections. The Roman surface fails to be an immersion as it contains cross-caps.