2004 AIME II Problem 8

Below is the professionally curated solution for Problem 8 of the 2004 AIME II, from LIVE by Po-Shen Loh. You can also try the full timed exam, view all 2004 AIME II solutions, or check the answer key.

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Concepts:factor countingprime factorizationstars and bars

Difficulty rating: 2450

8.

How many positive integer divisors of 200420042004^{2004} are divisible by exactly 20042004 positive integers?

Solution:

Since 2004=223167,2004 = 2^2 \cdot 3 \cdot 167, we have 20042004=24008320041672004,2004^{2004} = 2^{4008} \cdot 3^{2004} \cdot 167^{2004}, so its divisors are N=2i3j167kN = 2^i 3^j 167^k with i4008i \le 4008 and j,k2004.j, k \le 2004. Such an NN has (i+1)(j+1)(k+1)(i+1)(j+1)(k+1) divisors, so we need (i+1)(j+1)(k+1)=2004.(i+1)(j+1)(k+1) = 2004.

Every ordered triple of positive integers with product 20042004 yields admissible exponents, since each factor is at most 2004.2004. Counting prime by prime: the exponent 22 of the prime 22 is split among the three factors in (2+22)=6\binom{2+2}{2} = 6 ways by stars and bars, and each of the primes 33 and 167167 goes to one of the 33 factors.

The count is 633=54.6 \cdot 3 \cdot 3 = 54.

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