Particle A, at rest, decays into three or more particles: P1, P2, ..., Pn. Determine the maximum and minimum energies that P1 can have in such a decay, in terms of various masses. My solution: First of all, the decay should not occur if the rest mass of A would be smaller than the sum of the rest mass of P1, P2, ..., Pn. energy conservation. E1 minimum is the rest energy of P1 which is just created, with zero momentum. The rest of the energy is taken by the rest energies and momentum of P2, P3, etc. E1 max if E2+E3+...En is minimum, that is if P2 is at rest, P3 is at rest, ..., Pn is at rest. But this is false, because then, by conservation of momentum, P1 should also be at rest and would have E1 min, not maximum. At least two particles should have non zero momentum. E1 and another one. But then? I tried an analytical approach, but the calculus would need Mathematica. Any suggestions for a straighforward solution? Thanks.