Wolfram Research
Wolfram Mathworld: Hypocycloid
This clearly explains how a hypocycloid is obtained with great visual aids. Cartesian parametric formulas are given for a general hypocycloid. The last half of the article details how to determine specifics on properties of various...
Other
Old Dominion U.: Trochoids, Hypocycloids, & Epicycloids
Has discriptions of Cycloids, Trochoids, Hypocycloids, and Epicycloids are made.
University of St. Andrews (UK)
University of St. Andrews: Hypocycloid
Shows the graph and the parametric cartesian equation. The site also includes links to graphs of related curves and definitions of the associated curves.