Dirac equation for membranes
  Dirac's idea of taking the square root of constraints is applied to the case of extended objects concentrating on membranes in D=4 space-time dimensions. The resulting equation is Lorentz invariant and predicts an infinite hierarchy of positive and negative masses (tension). There are no tachyonic solutions. 
Maciej Trzetrzelewski
hep-th hep-ph math-ph math.MP