Find the smallest positive integer such that .
Notation : denotes the Euler's totient function .
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Obviously, since 1 2 0 and n are both integers, they can be factored in the form a ⋅ b ⋅ c … for factors a , b , c , … .
Now, since ϕ ( p k ) = p k − 1 ⋅ ( p − 1 ) for primes p , we should factor 120 into the form ( p a k a − 1 ⋅ ( p a − 1 ) ) ⋅ ( p b k b − 1 ⋅ ( p b − 1 ) … for some primes p a , p b … . Then, we get n = p a k a ⋅ p b k b … .
Now, since n is to be minimized, we can get the minimum value of n if there is only one prime factor of n , and k = 1 , since this gives us n = p for some prime p, and p − 1 = 1 2 0 . But, this implies that p = 1 2 1 , , and since 1 2 1 is not prime, we reject this solution.
Our next smallest value of n can be acquired f there is only one prime factor of n , and k = 2 , since this implies that n = p 2 for some prime p, and ( p ) ( p − 1 ) = 1 2 0 . But, since 120 cannot be factored into two consecutive integers, this idea is to be tossed into the trash bin.
Our next smallest value of n can be acquired when there are two prime factors of n , namely p a , p b and k a = k b = 1 , since this implies that n = p a ⋅ p b and ( p a − 1 ) ⋅ ( p b − 1 ) = 1 2 0 . Also, ∣ p a − p b ∣ must be minimized in order to minimize n . By factoring out 1 2 0 , we get that 1 2 0 = 1 2 ⋅ 1 0 = ( 1 3 − 1 ) ( 1 1 − 1 ) . This implies that n = 1 3 ⋅ 1 1 = 1 4 3 , which is our final answer.