Kramers-Kronig relations. For the classical anharmonic oscillator model of Eq. (1.4.20) show by explicit calculation that χ(2) (2ω;ω,ω) obeys the Kramers-Kronig relations in the form (1.7.12). Show also that χ(2) (ω1;ω3,-ω2) does not satisfy Kramers-Kronig relations.