Problem:
Let be a positive integer and let be a sequence of positive integers such that for every . The terms of the sequence are written one after another and in this way one obtains an infinite sequence of digits. Prove that for every positive integer there exists a positive integer such that the number formed by the first digits of the above sequence is divisible by .
Solution
Solution:
Let be an arbitrary positive integer. We shall prove that there exists a term of the sequence , whose decimal representation is obtained from that of by adding several digits from the right, i.e. the number is a "beginning" of that member.
Let be an index such that , where is a positive integer which is greater than the number of the digits of . Then
and obviously satisfies the above requirement.
Let , where . It is enough to prove the assertion of the problem for , where .
Let us consider the number
Here , where is the Euler function, is a positive integer such that , the number is greater than , and the number of 1's is .
Then is a "beginning" of some . Hence the sequence of the digits (formed by the terms of the sequence written one after another) looks like this:
where are the digits before . It is clear now that, depending on the remainder of modulo , we can add suitable digits from to in such a way that the resulting number is divisible by .