Relaxation Problem Number 1

p , q , r p, q, r are (not necessarily distinct) prime numbers and given that p q r pqr has m m positive distinct factors including 1 1 and itself. Find the sum of all possible values of m m .

Image Credit: Wikimedia Factor Tree


The answer is 18.

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1 solution

Jake Lai
Mar 18, 2015

Let the number of factors of n n be denoted σ 0 ( n ) \sigma_{0}(n) . Turns out that for n = i p i a i \displaystyle n = \prod_{i} p_{i}^{a_{i}} (Fundamental Theorem of Arithmetic), where p i < p i + 1 p_{i} < p_{i+1} are both primes and a i = 0 , 1 , 2 , a_{i} = 0, 1, 2, \ldots , σ 0 ( n ) = i ( a i + 1 ) \displaystyle \sigma_{0}(n) = \prod_{i} (a_{i}+1) .

Three possibilities:

p q r σ 0 ( p q r ) = σ 0 ( p 1 q 1 r 1 ) = ( 1 + 1 ) ( 1 + 1 ) ( 1 + 1 ) = 8 p \neq q \neq r \longrightarrow \sigma_{0}(pqr) = \sigma_{0}(p^{1}q^{1}r^{1}) = (1+1)(1+1)(1+1) = 8

p q = r σ 0 ( p q r ) = σ 0 ( p 1 q 2 ) = ( 1 + 1 ) ( 2 + 1 ) = 6 p \neq q = r \longrightarrow \sigma_{0}(pqr) = \sigma_{0}(p^{1}q^{2}) = (1+1)(2+1) = 6

p = q = r σ 0 ( p q r ) = σ 0 ( p 3 ) = 3 + 1 = 4 p = q = r \longrightarrow \sigma_{0}(pqr) = \sigma_{0}(p^{3}) = 3+1 = 4

Their sum is then 8 + 6 + 4 = 12 8+6+4 = \boxed{12} .

Nice, except 8+6+4 is not 12.

Joel Tan - 6 years, 2 months ago

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