Asked by Giulian

Let f(n) be the sum of the positive integer divisors of n. For how many values of n, where 1<n<25, is f(n) prime?
Answered by Cheerios!?
I don't know how to answer this but maybe this would help?
There are 18 possible values of n.
Hope this helps! :p
Answered by oobleck
f(2) = 1+2 = 3 ✅
f(3) = 1+3 = 4 ❌
f(4) = 1+2+4 = 7 ✅
f(5) = 1+5 = 6 ❌
and so on
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