I'm totally lost with this derivation of deterministic time dependent volatility in the BS equation. The screenshot shows the Appendix from Glenn Swindle (Valuation and Risk Mgmt in Energy...). Can anyone shed light on what's going on with the substitution in A.15 and how we get the expression for ds/dt? Or even point me to a reference that might have more details on how ds/dt was derived...would be deeply appreciated.