Here are my 5 cents.
2x2 matrix determinant:
that the exercise for the reader should be easy to implement
3x3 matrix determinant:
use a scalar three-dimensional product. This will require intelligent implementations of cross()and dot(). Recipes for them are widely available.
4x4 matrix determinant:
. :
template <class T>
inline T det(matrix<T, 4, 4> const& m) noexcept
{
auto const A(make_matrix<T, 2, 2>(m(0, 0), m(0, 1), m(1, 0), m(1, 1)));
auto const B(make_matrix<T, 2, 2>(m(0, 2), m(0, 3), m(1, 2), m(1, 3)));
auto const C(make_matrix<T, 2, 2>(m(2, 0), m(2, 1), m(3, 0), m(3, 1)));
auto const D(make_matrix<T, 2, 2>(m(2, 2), m(2, 3), m(3, 2), m(3, 3)));
return det(A - B * inv(D) * C) * det(D);
}
5x5 +:
.