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A221284
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Numbers n such that phi(m) = n^2 for some m.
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5
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1, 2, 4, 6, 8, 10, 12, 14, 16, 18, 20, 24, 26, 28, 30, 32, 36, 40, 42, 44, 48, 50, 52, 54, 56, 60, 64, 66, 68, 70, 72, 74, 80, 84, 88, 90, 94, 96, 100, 104, 108, 110, 112, 114, 116, 120, 124, 126, 128, 130, 132, 134, 136, 140, 144, 146, 148, 150, 156, 160, 162, 168, 170
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OFFSET
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1,2
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LINKS
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FORMULA
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Pollack and Pomerance show that n (log n)^0.0063 << a(n) << n (log n)^3.
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MATHEMATICA
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inversePhiSingle[(m_)?EvenQ] := Module[{p, nmax, n}, p = Select[Divisors[m] + 1, PrimeQ]; nmax = m*Times @@ (p/(p-1)); n = m; While[n <= nmax, If[EulerPhi[n] == m, Return[n]]; n++]; 0];
Reap[For[k = 1, k <= 200, k = k + If[k==1, 1, 2], If[inversePhiSingle[k^2] > 0, Print[k]; Sow[k]]]][[2, 1]] (* Jean-François Alcover, Dec 11 2018 *)
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PROG
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(PARI) is(n)=istotient(n^2)
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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STATUS
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approved
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