We've been busy generating new lessons, and are modifying the site organization slightly to accommodate the new content. In particular, there are new lessons on the way discussing difference and differential system descriptions for linear time-invariant systems, the Fourier series, Fourier transforms for periodic signals, and frequency-domain descriptions for systems described by difference and differential equations. So we've split the LTI Systems and Fourier Transforms category into two categories: Time Domain LTI Systems and Fourier Series and Transforms. Watch for the new lessons to begin appearing in these categories!