PROFESSOR P.P. DIVAKARAN

The focus of the lecture will be on the two main theorems on cyclic quadrilaterals stated by Brahmagupta (on the area and the lengths of the diagonals in terms of the sides) and later work on related ideas, culminating in their complete written proofs many centuries later. The starting point of the theory which led to these results was the idea of permutations of the sides and the invariance of certain properties under permutations. The ``third diagonal" of the title plays a key role in the working out of the theory.