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Number theory Difficulty 6.2 National Olympiad Prove it Balkan Mathematical Olympiad

Two positive integers mm and nn will be called anagrams, if each decimal digit aa appears as many times in the decimal representation of mm as in that of nn. 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 pp be a prime number such that its index modulo 1010 be equal to p1p-1 (i.e. the numbers 00, 11, 1010, \ldots, 10p210^{p-2} form a complete residue system modulo pp.) Let N(p)N(p) be the number 10p11p\frac{10^{p-1}-1}{p} with added leading zeroes in order to be a (p1)(p-1)-digit number. Then the numbers iN(p)iN(p) for 1ip11 \le i \le p-1, each one with added leading zeroes in order to be a (p1)(p-1)-digit number, are anagrams of N(p)N(p).

Indeed for 1ip11 \le i \le p-1 we can find kk such that 10ki(modp)10^k \equiv i \pmod p. Now iN(p)iN(p) is a period of the repeating decimal ip\frac{i}{p}, and hence of 10kp\frac{10^k}{p}. However the period of the latter is a cyclic permutation of the period of 1p\frac{1}{p}, which equals N(p)N(p).

The index of 1717 modulo 1010 is 1616. If leading zeroes were allowed, the numbers N(17)N(17), 2N(17)2N(17), 3N(17)3N(17) and 4N(17)4N(17) would provide a suitable example. To bypass the leading zeroes problem, we can glue to the left of each of these numbers a number ss with no leading zeroes which is an anagram of 3s3s. Since the index of 77 modulo 1010 is 66 and 7<107 < 10, a suitable example is N(7)=142857N(7) = 142857. Thus a possible example is given by the numbers 1016N(7)+iN(17)10^{16}N(7) + iN(17) for 1i41 \le i \le 4.

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