POLAR EQUATION OF INWARDLY SPIRALING SPIRAL

Answer 1

One such form would be
r = a e*(-b theta)

Whether the curve is spiraling in or out depends upon your point of view.

The "logartihmic spiral" a*e^(b theta) spirals outward as you move counterclockwise along it.

http://en.wikipedia.org/wiki/Logarithmic_spiral