Prove that there is a positive integer, not divisible by 10, whose 2011-th power has in its decimal expansion (at least) 2011 consecutive zeros immediately after its non-zero leading digit.
Solution
A number starts with the digit 1 followed by r zeros if and only if it can be written in the form 10r+s+A, with 0≤A<10s. If k>r≥0 we have 10r+s+10kA+A<10r+s+10k+s+10s<10k+s+1. Hence (10k+1)(10r+s+A)=10k+r+s+10r+s+10kA+A is a number which has at least r−1 zeros after its leading digit 1. By induction, we obtain for k>r≥0 and any 1≤m<r that (10k+1)m(10r+s+A) has at least r−m zeros after the leading digit 1. With A=1, s=1, r=k−1 we obtain that the number (10k+1)2011=(10k+1)2010(10k+1) has at least k−1−2010 zeros after the leading digit 1, provided that k>2011. Hence, any number 10k+1 with k≥4022 gives a solution.
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Source: MathNet,
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