(Q34+44) If an operator S can be written as S = TT, then we abbreviate S1/2 := T. In the mapping sequence x → Tx → TTx, clearly S1/2 “lies between” S0 and S1. We know that (AB)−1 = B−1A−1, so far existant, and so S−1 = T−1T−1. So we can abbreviate S−1/2 := T−1, and (S1/2)−1 = S−1/2. It is now clear that S−1/2SS−1/2 = T−1TTT−1 = Id.
It remains to show that S−1/2 is a limit of polynomials in S−1 in the strong operator topology.