Initialise 2x2 block-diagonal matrix

[stevengj]

@baptnz

I’m writing code using some linear algebra, where I need to set up a 6Nx6N matrix (called F), with a 2x2 block layout, each 3Nx3N block being itself composed of N^2 3x3 sub-blocks.
The initialisation of F should be such the diagonal of each 3Nx3N block is filled with ones.

Sounds like you just want:

I₃ = Matrix{T}(I, 3N, 3N)
F = [I₃ I₃; I₃ I₃]

or equivalently

F = diagm(0 => ones(T,6N), 3N => ones(T,3N), -3N => ones(T,3N))

or equivalently

F = kron(ones(T,2,2), I(3N))

All of these match the F constructed by your code above.