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2014 AMC 12B Problem 15

Problem 15 of 25IntermediateAlgebraNumber Theory

When p=k=16klnk,p = \sum_{k=1}^{6} k \ln k, the number epe^p is an integer. What is the largest power of 22 that is a factor of ep?e^p?

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Solution

Since klnk=ln(kk),k \ln k = \ln(k^k), the sum gives p=ln(k=16kk),p = \ln\left(\prod_{k=1}^{6} k^k\right), so ep=112233445566. e^p = 1^1 \cdot 2^2 \cdot 3^3 \cdot 4^4 \cdot 5^5 \cdot 6^6. The factors of 22 come from 222^2 (giving 22), 44=284^4 = 2^8 (giving 88), and 66=26366^6 = 2^6 \cdot 3^6 (giving 66). In total the exponent of 22 is 2+8+6=16.2 + 8 + 6 = 16. Thus, the correct answer is C.

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Concepts: logarithm · prime factorization

Problem text and solution from the LIVE past-contest archive. See also the AoPS wiki page for community solutions.