are (not necessarily distinct) prime numbers and given that
has
positive distinct factors including
and itself. Find the sum of all possible values of
.
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Let the number of factors of n be denoted σ 0 ( n ) . Turns out that for n = i ∏ p i a i (Fundamental Theorem of Arithmetic), where p i < p i + 1 are both primes and a i = 0 , 1 , 2 , … , σ 0 ( n ) = 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 = q = r ⟶ σ 0 ( p q r ) = σ 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
Their sum is then 8 + 6 + 4 = 1 2 .