Take a number z of x digits and consider any concatenation z = concat(y_1, y_2, ..., y_i) where y_1, y_2, ..., y_i have the same number of digits. Then let f(z) be the sum of the products y_1*y_2*...*y_i for all those concatenations. Sequence lists numbers z such that f(f(z)) = z. (see example.)

A317593

Take a number z of x digits and consider any concatenation z = concat(y_1, y_2, ..., y_i) where y_1, y_2, ..., y_i have the same number of digits. Then let f(z) be the sum of the products y_1*y_2*...*y_i for all those concatenations. Sequence lists numbers z such that f(f(z)) = z. (see example.)