Before revision: In this paper we present invariants of the 1d family of 3-periodics (triangular orbits) in an Elliptic Billiard (EB) , previously mentioned [ 14 ], including: (i) explicit Expressions classic invariants: perimeter and Joachimsthal's quantity; (ii) consevation of Inradius-to-Circumradius; (iii) stationarity of the Cosine Circle of the Excentral Triangle and the fact that it is exterior to the EB. Finally we present an entirely new N=3 invariant .
After revision: In this paper we present invariants of the 1d family of 3-periodics (triangular orbits) in an Elliptic Billiard (EB) , previously mentioned [ 14 ], stemming from both experimental and theoretical work. These include relationships between radii, angles and areas of triangular members of the family, as well as a special stationary circle. Here we present the proofs promised there as well as a few new related facts .