 Complete the following phrase:  In the case of differential equations, as in the case of matrix equations, whenever the right-hand side of an equation is zero i.e., the forcing function  forcing vector is zero, the equation may still admit a non-trivial solution, known in applied mathematics as an eigenfunction solution, in physics as a natural mode solution and in electrical circuit theory as the zero-input response. This is a concept that spans a wide range of physical disciplines. Common physical examples of resot natural modes would include the resot vibrational modes of stringed instruments 1D, percussion instruments 2D or the former 