Boost LA

#include <boost/la/matrix_mul_scalar.hpp>

#include <boost/la/deduce_matrix.hpp>

namespace
boost
    {
    namespace
    la
        {
        //Only enabled if: is_matrix<A>::value
        template <class A>
        typename deduce_matrix<A>::type
        operator*( A const & a, matrix_traits<A>::scalar_type s );
        }
    }