Maths Olympiad Prep

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Algebra Difficulty 5.2 AIME, harder Prove it Ireland

Prove that there is a positive integer, not divisible by 1010, whose 20112011-th power has in its decimal expansion (at least) 20112011 consecutive zeros immediately after its non-zero leading digit.

Solution

A number starts with the digit 11 followed by rr zeros if and only if it can be written in the form 10r+s+A10^{r+s} + A, with 0A<10s0 \le A < 10^s. If k>r0k > r \ge 0 we have 10r+s+10kA+A<10r+s+10k+s+10s<10k+s+110^{r+s} + 10^k A + A < 10^{r+s} + 10^{k+s} + 10^s < 10^{k+s+1}. Hence (10k+1)(10r+s+A)=10k+r+s+10r+s+10kA+A(10^k + 1)(10^{r+s} + A) = 10^{k+r+s} + 10^{r+s} + 10^k A + A is a number which has at least r1r-1 zeros after its leading digit 11. By induction, we obtain for k>r0k > r \ge 0 and any 1m<r1 \le m < r that (10k+1)m(10r+s+A)(10^k + 1)^m (10^{r+s} + A) has at least rmr-m zeros after the leading digit 11. With A=1A = 1, s=1s = 1, r=k1r = k-1 we obtain that the number (10k+1)2011=(10k+1)2010(10k+1)(10^k + 1)^{2011} = (10^k + 1)^{2010} (10^k + 1) has at least k12010k-1-2010 zeros after the leading digit 11, provided that k>2011k > 2011. Hence, any number 10k+110^k + 1 with k4022k \ge 4022 gives a solution.

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