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diff --git a/src/main/scala/sims/dynamics/RegularPolygon.scala b/src/main/scala/sims/dynamics/RegularPolygon.scala
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+/*
+ * Simple Mechanics Simulator (SiMS)
+ * copyright (c) 2009 Jakob Odersky
+ * made available under the MIT License
+*/
+
+package sims.dynamics
+
+import math._
+import sims.geometry._
+
+/**A regular polygon with <code>n</code> sides whose excircle has a radius <code>radius</code>.
+ * @param n nmber of sides.
+ * @param radius radius of the excircle
+ * @param density density of this regular polygon
+ * @throws IllegalArgumentException if <code>n</code> is smaller than 3 */
+case class RegularPolygon(n: Int, radius: Double, density: Double) extends Shape with ConvexPolygon{
+  require(n >= 3, "A polygon must have at least 3 sides.")
+  
+  /**Height of one of the constituting triangles.*/
+  private val h: Double = radius * cos(Pi / n)
+  /**Half width of one of the constituting triangles.*/
+  private val b: Double = radius * sin(Pi / n)
+  
+  def halfDiags = (for (i: Int <- (0 until n).toArray) yield (Vector2D(0, radius) rotate (2 * Pi * i / n))) map (_ rotate rotation)
+  
+  def vertices = for (h <- halfDiags) yield pos + h
+  
+  val volume = n * h * b
+  
+  /**Moment of inertia of one of the constituting triangles about the center of this polygon.*/
+  private val Ic: Double = density * b * (3 * b + 16) * h * h * h * h / 54
+  
+  val I = n * Ic
+}