202 5. ENERGY, MONOTONICITY, AND BREATHERSD2.3.2. Deriving the monotonicity of :F from a pointwise estimate. This
second approach to the energy monotonicity formula is based on the point-
wise formula (5.43), which is a simpler version of the evolution equation for
Perelman's backward Harnack quantity (6.22).
Let (g (t), f (t)) be a solution to (5.34)-(5.35). Let u = e-f and
(5.42) V ~ (2/:lf - IV f 12 + R)u = Rmu,
where Rm is the modified scalar curvature defined by (5.15),^8 so that:F= JM Vdμ.
LEMMA 5.19 (Bochner-type formula for V). If (g (t), f (t)) is a solution
to (5.34)-(5.35) and if u = e-f, then we have the pointwise differential
equality:
(5.43)This calculation, which we carry out below, is in a similar spirit to that
of the calculations for the differential Harnack quantities considered in §10
of Chapter 5 in Volume One and Part II of this volume. To obtain (5.41)
from the lemma, we computePROOF OF THE LEMMA. Using definition (5.42) and gij gtrfj = 0, a di-
rect calculation shows that:t Rm= :t (2/:lf - IV fl2 + R)
= 4~jViVjf + 2/:l ( ~{) - 2RijVdVjf-2V (~{)·VJ+~~
= 4Rij ViVjf - /:l(2/:lf - IV !1^2 + R) + !:llV !1^2 + 2V !:lf. v f
-2RijVdVjf - 2V(IVfl^2 - R)Vf + ~~ - !:lR.
From the above we have(%t + /j.) Rm= 2IRij + ViVjfl^2 + 2VRm. Vf.
(^8) The above V is not to be confused with our earlier V, which was the trace of the
variation v of g.