cross ratio: for example, four pts A,B,C,D on a line, then the ~ of them is AD*BC/(AC*BD)
adjoint: the ~ of the linear operator, as stated in functional analysis
n-gons: form sqrt(5), then 72degree is constructible, henceforth 18degree (a quater) and 12degree (minus 60degree), the result of 17-gon is due to Gauss
green function: interpret as in pde
order: there is something wrong with the definition