If M2n is a cohomology ℂPn
and p is a prime, let Dp(M2n) be the set of positive integers d such that
d ∈ Dp(M2n) if there exists a diffeomorphism of M2n of order p fixing an orientable,
codimension-2 submanifold of degree d. If p = 2 or n is odd, then 1 ∈ Dp(M2n)
implies that Dp(M2n) = {1}. The case p odd and n even is also investigated. If M4m
is a homotopy ℂP2m and m≢0,4, or 7 (mod8), then 1 ∈ D3(M4m) implies that
D3(M4m) = {1}.