Dipped into tensor calculus

Just finished doing calculations for tomorrow. I had to perform the procedure known as tensor contraction on the electromagnetic tensor to prove that it gives a Lorentz-invariant scalar. Never thought that people at the first year in uni should do such things. I had to spend half an hour to understand how to make this contraction. Because electromagnetic tensor is 2-rank tensor after contracting it with itself one obtains 0-rank tensor that is scalar. So I found out the actual value which equals to H^2-E^2 (H-magnetic field, E-electric field).