Some time ago I read (maybe a wikipedia page?) about work regarding a form of a real function presented as sum of convolutions of different order, where "normal" convolution
$$f(x)=\int k(x,x')g(x')dx'$$
is considered as an first order, similar what $f'(x_0)(x-x_0)$ is to Taylor series. IIRC higher-orders had product of functions, maybe kernels or input functions, within integral. Does anyone perhaps know what this may be?