The sigmoid link introduces the named-boundary register. Only the midpoint is exact; the other sigmoid entries stay symbolic.
highlighted = computed this step
The sigmoid link
The sigmoid maps a score into the open unit interval. Its formula contains an exponential, so the curve is a named boundary guide.
σ(z)=1+e−z1
The one exact midpoint
When z=0, sigmoid equals 1/2 exactly. The other sampled points are named symbols, not decimal values or exact claims.
σ(0)=1/2
Summary
Sigmoid and logarithm are named boundary operations here; the rendered structure is exact, but no decimal value is pinned for them. The threshold decision from the previous lesson did not need sigmoid.