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Algebra Difficulty 5.2 AIME, harder Find the answer

Mary has a sequence m2,m3,m4,m_{2}, m_{3}, m_{4}, \ldots, such that for each b2,mbb \geq 2, m_{b} is the least positive integer mm for which none of the base-bb logarithms logb(m),logb(m+1),,logb(m+2017)\log _{b}(m), \log _{b}(m+1), \ldots, \log _{b}(m+2017) are integers. Find the largest number in her sequence.

A number or a short expression. Spacing and $ signs are ignored.

Solution

It is not difficult to see that for all of the logarithms to be non-integers, they must lie strictly between nn and n+1n+1 for some integer nn. Therefore, we require bn+1bn>2018b^{n+1}-b^{n}>2018, and so mb=bn+1m_{b}=b^{n}+1 where nn is the smallest integer that satisfies the inequality. In particular, this means that bnbn12018b^{n}-b^{n-1} \leq 2018. Note that m2=211+1=2049(m_{2}=2^{11}+1=2049\left(\right. since 212211=2048>2018)\left.2^{12}-2^{11}=2048>2018\right) and m3=37+1=2188m_{3}=3^{7}+1=2188 (since 3837=4374>20183^{8}-3^{7}=4374>2018 ). we now show that 2188 is the maximum possible value for mbm_{b}. If n=0n=0, then mb=1+1=2m_{b}=1+1=2. If n=1n=1, then b12018b-1 \leq 2018 and thus mb=b+12020m_{b}=b+1 \leq 2020. If n=2n=2, then b2b2018b^{2}-b \leq 2018, which gives b45b \leq 45, and thus mb=b2+12018+b+12065m_{b}=b^{2}+1 \leq 2018+b+1 \leq 2065. If n=3n=3, then b3b22018b^{3}-b^{2} \leq 2018, which gives b12b \leq 12, and thus mb=b3+1123+1=1729m_{b}=b^{3}+1 \leq 12^{3}+1=1729. If n=4n=4, then b4b32018b^{4}-b^{3} \leq 2018, which gives b6b \leq 6, and thus mb=b4+164+1=1297m_{b}=b^{4}+1 \leq 6^{4}+1=1297. It then remains to check the value of m4m_{4} and m5m_{5}. Indeed, m4=45+1=1025m_{4}=4^{5}+1=1025 and m5=54+1=626m_{5}=5^{4}+1=626, so no values of mbm_{b} exceeds 2188.

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Source: Omni-MATH, licensed Apache-2.0. Statement and solution reproduced as published; topic and difficulty added by this site.