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digits in sequence of powers of 2

Source: Spanish Mathematical Olympiad 1968 P7

December 4, 2022
number theoryDigitspower of 2

Problem Statement

In the sequence of powers of 22 (written in the decimal system, beginning with 21=22^1 = 2) there are three terms of one digit, another three of two digits, another three of 33, four out of 44, three out of 55, etc. Clearly reason the answers to the following questions: a) Can there be only two terms with a certain number of digits? b) Can there be five consecutive terms with the same number of digits? c) Can there be four terms of n digits, followed by four with n+1n + 1 digits? d) What is the maximum number of consecutive powers of 22 that can be found without there being four among them with the same number of digits?