Prove/disprove the following inequalities.
If MMM and kkk are natural numbers, then
1)∑n=1Mϕ(n)≥1∑n=1Mϕ(n)1) \sum_{n=1}^{M} \phi(n) \geq \frac{1}{\sum_{n=1}^{M} \phi (n)}1)n=1∑Mϕ(n)≥∑n=1Mϕ(n)1
2)∑n=1Mϕ(n)≥∑n=1Mϕ(ϕ(n))2)\sum_{n=1}^{M} \phi(n) \geq \sum_{n=1}^{M} \phi(\phi(n))2)n=1∑Mϕ(n)≥n=1∑Mϕ(ϕ(n))
Notation: ϕ(⋅)\phi(\cdot) ϕ(⋅) denotes Euler's totient function.
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Lemma:ϕ(nk)≥ϕ(n)k\phi(n^k)≥\phi(n)^kϕ(nk)≥ϕ(n)k
proof:ϕ(nk)=nk−1ϕ(n)≥ϕk(n)→nk−1≥ϕk−1(n)→n≥ϕ(n)\phi(n^k)=n^{k-1}\phi(n)≥\phi^k(n)\to n^{k-1}≥\phi^{k-1}(n)\to n≥\phi(n)ϕ(nk)=nk−1ϕ(n)≥ϕk(n)→nk−1≥ϕk−1(n)→n≥ϕ(n) which we know is true. the result follows
3.n≥ϕ(n)n≥\phi(n)n≥ϕ(n) let n=ϕ(x)n=\phi(x)n=ϕ(x) the ϕ(x)≥ϕ(ϕ(x))\phi(x)≥\phi(\phi(x))ϕ(x)≥ϕ(ϕ(x)) and the result follows.
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right, exactly. The only non-trivial one is ϕ(nk)=nk−1ϕ(n)\phi(n^k)=n^{k-1}\phi(n)ϕ(nk)=nk−1ϕ(n)
Darn I worked so hard to type the whole solution.
Thank god I started from the bottom, otherwise I would've worked harder, only to receive nothing :P
I thought about them during my morning shower, and they all appear to be true. Want any proofs? They are all one liners...
Morning shower is the best place to think abt maths , ;) , Actually I thought abt them and their proofs , in my morning shower
Q3. We will prove that ϕ(n)>ϕ(ϕ(n))\phi (n) > \phi (\phi (n))ϕ(n)>ϕ(ϕ(n))
ϕ(n)=n×(1−1a1)×(1−1a2)×⋯×1ak\phi (n) = n \times (1- \dfrac {1}{a_1}) \times (1- \dfrac {1}{a_2}) \times \cdots \times \dfrac {1}{a_k}ϕ(n)=n×(1−a11)×(1−a21)×⋯×ak1 where n=a1p1a2p2⋯akpkn = a_1 ^{p_1} a_2 ^{p_2}\cdots a_k ^{p_k}n=a1p1a2p2⋯akpk where a1,a2....,aka_1,a_2...., a_ka1,a2....,ak are prime numbers, and p1,p2...,pkp_1,p_2..., p_kp1,p2...,pk are positive integers.
Now, ϕ(ϕ(n))=ϕ(n×(1−1a1)×(1−1a2)×⋯×1ak))\phi (\phi (n)) = \phi (n \times (1- \dfrac {1}{a_1}) \times (1- \dfrac {1}{a_2}) \times \cdots \times \dfrac {1}{a_k}))ϕ(ϕ(n))=ϕ(n×(1−a11)×(1−a21)×⋯×ak1))
We observe that ϕ(n)<n\phi (n) < nϕ(n)<n , because it is multiplied by fractions less than 1.
Thus , we show that ϕ(ϕ(n))≤ϕ(n)\phi (\phi (n)) \leq \phi (n)ϕ(ϕ(n))≤ϕ(n) , thus the inequality holds true.
Small correction : ϕ(n)≤n\phi(n)\leq nϕ(n)≤n since you have equality for n=1n=1n=1
Oops yeah, thanks :D
Nicely done Mehul !
Thanks Chinmay! :D
I was about to write the other proofs as well, but Aareyan beat me ;)
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Lemma:ϕ(nk)≥ϕ(n)k
proof:ϕ(nk)=nk−1ϕ(n)≥ϕk(n)→nk−1≥ϕk−1(n)→n≥ϕ(n) which we know is true. the result follows
3.n≥ϕ(n) let n=ϕ(x) the ϕ(x)≥ϕ(ϕ(x)) and the result follows.
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right, exactly. The only non-trivial one is ϕ(nk)=nk−1ϕ(n)
Darn I worked so hard to type the whole solution.
Thank god I started from the bottom, otherwise I would've worked harder, only to receive nothing :P
I thought about them during my morning shower, and they all appear to be true. Want any proofs? They are all one liners...
Log in to reply
Morning shower is the best place to think abt maths , ;) , Actually I thought abt them and their proofs , in my morning shower
Q3. We will prove that ϕ(n)>ϕ(ϕ(n))
ϕ(n)=n×(1−a11)×(1−a21)×⋯×ak1 where n=a1p1a2p2⋯akpk where a1,a2....,ak are prime numbers, and p1,p2...,pk are positive integers.
Now, ϕ(ϕ(n))=ϕ(n×(1−a11)×(1−a21)×⋯×ak1))
We observe that ϕ(n)<n , because it is multiplied by fractions less than 1.
Thus , we show that ϕ(ϕ(n))≤ϕ(n) , thus the inequality holds true.
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Small correction : ϕ(n)≤n since you have equality for n=1
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Oops yeah, thanks :D
Nicely done Mehul !
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Thanks Chinmay! :D
I was about to write the other proofs as well, but Aareyan beat me ;)