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We review the explanation for resonant neutron scattering experiments in$YBa_2Cu_3O_{6+\delta}$ and $Bi_2 Sr_2 Ca_2 O_{8+\delta}$ materials from thepoint of view of triplet excitation in the particle-particle channel, the piexcitation. Relation of these resonances to the superconducting condensationenergy and their role in stabilizing the superconducting state is discussed.Due to the superconducting fluctuations, the pi resonance may appear as a broadpeak above Tc. Analogue problem to the pi excitation for the case of s-wavepairing, the eta excitation, is considered in the strong coupling limit with anemphasis on the resonance precursors in a state with no long range order.Comment: 9 pages, 2 eps fig

Resonant neutron scattering on the high Tc cuprates and π and η excitations of the t-J and Hubbard models