The Math
Beginning:
F=0, S=2.  P(i)=0
For insert with no rehashing
C(i+1)=1
P(i+1)-P(i)=2 (added non-empty slot)
For insert with rehashing of N items
C(i+1)=1+N
P(i+1)-P(i)=2-N (before, F=N, S=2N.  After, S=4N)