On Rotations as Spin Matrix Polynomials
  Recent results for rotations expressed as polynomials of spin matrices are derived here by elementary differential equation methods. Structural features of the results are then examined in the framework of biorthogonal systems, to obtain an alternate derivation. The central factorial numbers play key roles in both derivations. 
T. L. Curtright and T. S. Van Kortryk
math-ph hep-th math.MP quant-ph