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Number theory Difficulty 4.7 AIME Find the answer

Find the sum d=120122012d\sum_{d=1}^{2012}\left\lfloor\frac{2012}{d}\right\rfloor.

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

Solution

Note that the number of integers between 1 and 2012 that have nn as a divisor is 2012n\left\lfloor\frac{2012}{n}\right\rfloor. Therefore, if we sum over the possible divisors, we see that the sum is equivalent to d=120122012d\sum_{d=1}^{2012} \left\lfloor\frac{2012}{d}\right\rfloor. This can be approximated by d=120122012d=2012d=120121d2012ln(2012)\sum_{d=1}^{2012} \frac{2012}{d}=2012 \sum_{d=1}^{2012} \frac{1}{d} \approx 2012 \ln (2012). As it turns out, 2012ln(2012)153002012 \ln (2012) \approx 15300, which is worth 18 points. Using the very rough approximation ln(2012)7\ln (2012) \approx 7 still gives 14 points.

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