Def. Integral equation（积分方程）. An equation in which the unknown function occurs under an integral sign. Examples of integral equations are Fredholm’s integral equations of the first, second and third kinds:
1. Fredholm’s integral equation of the first kind. The equation



in which g and K are two given functions and f is the unknown function.
2. Fredholm’s integral equation of the second kind. The equation


in which h and K are two given functions and f is the unknown function.

3. Fredholm’s integral equation of the third kind. The equation


in which g, h and K are given functions and f is the unknown function.

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