Two positive integers and will be called anagrams, if each decimal digit appears as many times in the decimal representation of as in that of . Is it possible to find four different positive integers such that each of them is an anagram of the sum of the other three?
Solution
Let be a prime number such that its index modulo be equal to (i.e. the numbers , , , , form a complete residue system modulo .) Let be the number with added leading zeroes in order to be a -digit number. Then the numbers for , each one with added leading zeroes in order to be a -digit number, are anagrams of .
Indeed for we can find such that . Now is a period of the repeating decimal , and hence of . However the period of the latter is a cyclic permutation of the period of , which equals .
The index of modulo is . If leading zeroes were allowed, the numbers , , and would provide a suitable example. To bypass the leading zeroes problem, we can glue to the left of each of these numbers a number with no leading zeroes which is an anagram of . Since the index of modulo is and , a suitable example is . Thus a possible example is given by the numbers for .