View source for Hammerstein equation

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A non-linear integral equation of the type

$$\phi(x)+\int\limits_a^bK(x,s)f[s,\phi(s)]ds=0,\quad a\leq x\leq b,$$

where $K(x,s)$ and $f(x,s)$ are given functions, while $\phi(x)$ is the unknown function. Named after A. Hammerstein [[#References|[1]]], who considered the case where $K(x,s)$ is a symmetric and positive Fredholm kernel, i.e. all its eigen values are positive. If, in addition, the function $f(x,s)$ is continuous and satisfies the condition

$$|f(x,s)|\leq C_1|s|+C_2,$$

where $C_1$ and $C_2$ are positive constants and $C_1$ is smaller than the first eigen value of the kernel $K(x,s)$, the Hammerstein equation has at least one continuous solution. If, on the other hand, $f(x,s)$ happens to be a non-decreasing function of $s$ for any fixed $x$ from the interval $(a,b)$, Hammerstein's equation cannot have more than one solution. This property holds also if $f(x,s)$ satisfies the condition

$$|f(x,s_1)-f(x,s_2)|\leq C|s_1-s_2|,$$

where the positive constant $C$ is smaller than the first eigen value of the kernel $K(x,s)$. A solution of the Hammerstein equation may be constructed by the method of successive approximation (cf. [[Sequential approximation, method of|Sequential approximation, method of]]).

====References====
<table><TR><TD valign="top">[1]</TD> <TD valign="top">  A. Hammerstein,   "Nichtlineare Integralgleichungen nebst Anwendungen"  ''Acta Math.'' , '''54'''  (1930)  pp. 117–176</TD></TR><TR><TD valign="top">[2]</TD> <TD valign="top">  F.G. Tricomi,   "Integral equations" , Dover, reprint  (1985)</TD></TR><TR><TD valign="top">[3]</TD> <TD valign="top">  M.M. Vainberg,   "Variational methods for the study of nonlinear operators" , Holden-Day  (1964)  (Translated from Russian)</TD></TR><TR><TD valign="top">[4]</TD> <TD valign="top">  M.A. Krasnosel'skii,   "Topological methods in the theory of nonlinear integral equations" , Pergamon  (1964)  (Translated from Russian)</TD></TR><TR><TD valign="top">[5]</TD> <TD valign="top">  N.S. Smirnov,   "Introduction to the theory of integral equations" , Moscow-Leningrad  (1936)  (In Russian)</TD></TR></table>