Hi Anh Tuan, To^i co' ca'i rule na`y anh xa`i thu+? nhe'. -TION thi` la` Integral cu?a Root. -TIVITY la` derivative. Co' the^? la^'y expectation value de^? co' NESS cha(ng? Nho+` ca'c anh nghi~ ho^. Just an inspring thought that may need a correction. Cheers Aiviet