Positive Lower Density of Collatz Predecessors source
Every fixed positive target not divisible by 3 has ordinary Collatz predecessors of positive lower natural density. A counterexample would force positive lower density of nonconvergence; upper-density-one convergence would imply universal convergence, but that premise is not proved.
The companion source ZIP contains the narrowed extension, not the complete assembled library. Reproduction also requires the unchanged, hash-pinned supporting source release v2.1. The complete assembled import closure was checked privately; it is not republished here as raw Lean files or a second ZIP.
Line counts exclude blank lines; comments and documentation count. Counts cover the complete checked assembled closure, not the smaller downloadable extension. Mathlib and other third-party dependencies are excluded. Source size is not a proof-quality measure.