[eigen] Using Triangular Views

[Brad Bell <bradbell@xxxxxxxxxx>]

How does one multiply a lower triangular matrix times a lower triangular 
matrix and avoid multiplying the zeros in the lower triangular matrix ?

An attempt at this (see below) does not seem to compile when 
MULTIPLY_VIEW is non-zero: To be specific, I get the error message:

error: no match for ‘operator*’ (operand types are ‘const 
Eigen::TriangularView<Eigen::Matrix<double, 2, 2>, 1u>’ and ‘const 
Eigen::TriangularView<Eigen::Transpose<Eigen::Matrix<double, 2, 2> >, 2u>’)
    matrix B = view_L * view_L.transpose();

when I try to compile the program:
# include <Eigen/Core>
# include <iostream>

// code does not compile if you select MULTIPLY_VIEW equal to 1
# define MULTIPLY_VIEW 0

int main()
{   typedef Eigen::Matrix<double, 2, 2> matrix;
    using Eigen::Lower;
    using Eigen::TriangularView;

    matrix A;
    A <<
    1, 2,
    3, 4;
    //
    const TriangularView<matrix, Lower> view_L = A.triangularView<Lower>();
    matrix L = view_L;
    //
# if MULTIPLY_VIEW
    matrix B = view_L * view_L.transpose();
# else
    matrix B = view_L * L.transpose();
# endif
    //
    std::cout << "B =\n" << B << "\n";

    return 0;
}