Recently, a class of new solutions have been derived for a number of soliton equations using Hirota's bilinear forms of these soliton equations (S. Oishi: J. Phys. Soc. Jpn. 47 (1979) 1341). These solutions express solitons in a background of ripples, and are named generalized soliton solutions. In this paper, it is shown that the generalized soliton solutions for the Korteweg-de Vries equation and the Kadomtsev-Petviashvili equation can be transformed into a form of Fredholm's determinants of the Gel'fand-Levitan-Marchenko integral equation. Using this result, relationship between Hirota's method and the inverse spectral method is clarified. Moreover, it is also shown that the initial value problems for these two equations can be solved using their generalized soliton solutions.
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