Eliminating the CPT-Odd f Coefficient from the Lorentz-Violating   Standard Model Extension
  The fermionic f coefficient in the Lorentz-violating standard model extension presents a puzzle. Thus far, no observable quantity that depends upon f has ever been found. We show that this is because f is actually unnecessary. It has absolutely no effects at leading order and can be completely absorbed into other coefficients of the theory by a redefinition of the field. 
B. Altschul