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Multiples with small digits

Problem 303 Published on Saturday, 25th September 2010, 10:00 pm; Solved by 2877;
Difficulty rating: 35%

For a positive integer n, define f(n) as the least positive multiple of n that, written in base 10, uses only digits ≤ 2.

Thus f(2)=2, f(3)=12, f(7)=21, f(42)=210, f(89)=1121222.

Also, p303_formula100.gif.

Find p303_formula10000.gif.