Relativity: a quick introduction []
A quick introduction to Special Relativity with a derivation of

Mach's Principle and Evolution of the Universe []
Within the framework of Mach's principle and of D.W. Sciama's physical interpretation of it, a quantitative evaluation of the inertial mass as a function of the density of the universe and of the epoch is obtained. The calculation is somewhat similar to the solution of Olbers's paradox. The result shows that the inertial mass varies as the universe expands except for a flat, Euclidean universe. This leads to insights on the Principle of Equivalence.