Conservative Hénon stability boundary ($\det DF=1(b=1)$)

At $b=1$ the Hénon map has unit Jacobian determinant. The upper fixed point is linearly elliptic for $-1<\mu <3$, and its newborn two-cycle is elliptic for $3<\mu <4$. These are neutral multiplier statements, not attraction. The endpoints have double multipliers $+1$ and $-1$. Constant-area scaling also excludes a simple invariant closed curve enclosing positive area when $b>0$ and $b\ne 1$: invariance of its interior would require both $\mathrm{area}(F(D))=b\mathrm{area}(D)$ and $F(D)=D$. Thus the $b=1$ elliptic stability boundary is a conservative degeneracy, not a generic dissipative Neimark-Sacker bifurcation. Arbitrary non-area-preserving perturbations can change this behavior even though generic fold and flip curves persist.