The Klaassen aggregation enable us to reduce a binomial lattice of prices into two-point one-period model with larger period and it is consistent with the risk-neutral pricing principle. In general the aggregation is a computing backward procedure. Nevertheless in case of the Pedersen-Shiu-Thorlacius term structure model (which is the generalization of the Ho-Lee model) the analytical aggregation is possible. In the paper the bond prices and the pseudoprobabilities are obtained in the analytical forms. In the aggregated model the bond prices are proved to be restricted to an interval. Also the aggregated perturbation functions are deduced and they are compared to the perturbation functions in the one-period model. .
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