Number theoryDifficulty 5.5AIME, harderFind the answer
For a positive integer n, denote by τ(n) the number of positive integer divisors of n, and denote by ϕ(n) the number of positive integers that are less than or equal to n and relatively prime to n. Call a positive integer n good if φ(n)+4τ(n)=n. For example, the number 44 is good because φ(44)+4τ(44)=44. Find the sum of all good positive integers n.
A number or a short expression. Spacing and $ signs are ignored.
Solution
We claim that 44,56,72 are the only good numbers. It is easy to check that these numbers work. Now we prove none others work. First, remark that as n=1,2 fail so we have φ(n) is even, thus n is even. This gives us φ(n)≤n/2. Now remark that τ(n)<2n, so it follows we need n/2+8n>n⟹n≤256. This gives us a preliminary bound. Note that in addition we have 8τ(n)>n. Now, it is easy to see that powers of 2 fail. Thus let n=2ap1b where p1 is an odd prime. From 8τ(n)>n we get 8(a+1)(b+1)>2ap1b≥2a3b from which we get that (a,b) is one of (1,1),(1,2),(1,3),(2,1),(2,2),(3,1),(3,2),(4,1) Remark that p1≤∗∗2a8(a+1)(b+1). From this we can perform some casework: - If a=1,b=1 then p1−1+16=2p1 but then p=15, absurd. - If a=1,b=2 then we have p1≤5 which is obviously impossible. - If a=1,b=3 then p1≤4 which is impossible. - If a=2,b=1 then p1≤12 and it is easy to check that p1=11 and thus n=44 is the only solution. - If a=2,b=2 then p1≤4 which is impossible. - If a=3,b=1 then p1≤8 and only p1=7 or n=56 works. - If a=3,b=2 then p1≤3 and p1=3,n=72 works. - If a=4,b=1 then p1≤1 which is absurd. Now suppose n is the product of 3 distinct primes, so n=2ap1bp2c so we have 8(a+1)(b+1)(c+1)>2a3b5c then we must have (a,b,c) equal to one of (1,1,1),(1,2,1),(2,1,1),(3,1,1) Again, we can do some casework: - If a=b=c=1 then 8τ(n)=64>2p1p2 but then p1=3,p2=5 or p1=3,p2=7 is forced neither of which work. - If a=1,b=2,c=1 then 8τ(n)=96>2p12p2 but then p1=3,p2=5 is forced which does not work. - If a=2,b=1,c=1 then 8τ(n)=96>4p1p2 forces p1=3,p2=5 or p1=3,p2=7 neither of which work. - If a=3,b=1,c=1 then 8τ(n)=108>8p1p2 which has no solutions for p1,p2. Finally, take the case where n is the product of at least 4 distinct primes. But then n≥2⋅3⋅5⋅7=210 and as 2⋅3⋅5⋅11>256, it suffices to check only the case of 210 . But 210 clearly fails, so it follows that 44,56,72 are the only good numbers so we are done.
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