olymmath

RU-ALLRUSSIAN 1991 Problem 17

    A sequence of positive integers is constructed as follows. If the last digit of an\mathbf{a}_{\mathrm{n}} is greater than 5, then an+1\mathbf{a}_{\mathrm{n} + 1} is 9an9\mathbf{a}_{\mathrm{n}} . If the last digit of an\mathbf{a}_{\mathrm{n}} is 5 or less and an\mathbf{a}_{\mathrm{n}} has more than one digit, then an+1\mathbf{a}_{\mathrm{n} + 1} is obtained from an\mathbf{a}_{\mathrm{n}} by deleting the last digit. If an\mathbf{a}_{\mathrm{n}} has only one digit, which is 5 or less, then the sequence terminates. Can we choose the first member of the sequence so that it does not terminate?

    Dataset text NuminaMath olympiads. Quoted with source.

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