[...] It is a mathematical fact that the constant correlation matrix that has all off-diagonal entries equal to ρ is positive semidefinite for all values of ρ in the range [0,1]. Notice that this correlation matrix is a special case of a Toeplitz correlation structure. For example, toeplitz({1 .2 .2 .2}) is a valid correlation matrix. This is good to keep in mind, because sometimes it is hard to generate a positive definite matrix …