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A253215 a(n) is the greatest positive integer m such that phi(m) <= n where phi is Euler's totient function. 2
2, 6, 6, 12, 12, 18, 18, 30, 30, 30, 30, 42, 42, 42, 42, 60, 60, 60, 60, 66, 66, 66, 66, 90, 90, 90, 90, 90, 90, 90, 90, 120, 120, 120, 120, 126, 126, 126, 126, 150, 150, 150, 150, 150, 150, 150, 150, 210, 210, 210, 210, 210, 210, 210, 210 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

LINKS

Jean-François Alcover, Table of n, a(n) for n = 1..1000

MathOverflow, The maximum of the preimage of [1,x] through Euler's totient function

Eric W. Weisstein's mathWorld, Totient Function

Wikipedia, Euler's totient function

MATHEMATICA

inversePhi[m_?EvenQ] := Module[{p, nmax, n, nn}, p = Select[Divisors[m]+1, PrimeQ]; nmax = m*Times @@ (p/(p-1)); n = m; nn = {}; While[n <= nmax, If[EulerPhi[n] == m, AppendTo[nn, n]]; n++]; nn]; a[1] = 2; a[n_?OddQ] := a[n-1]; a[n_] := a[n] = Module[{m}, m = inversePhi[n] // Max; If[m > a[n-1], m, a[n-1]]]; Table[a[n], {n, 1, 100}]

CROSSREFS

Cf. A000010, A007614, A032447.

Sequence in context: A071892 A064797 A053319 * A075779 A241301 A140880

Adjacent sequences:  A253212 A253213 A253214 * A253216 A253217 A253218

KEYWORD

nonn

AUTHOR

Jean-François Alcover, Jan 08 2015

STATUS

approved

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Last modified November 13 12:45 EST 2019. Contains 329094 sequences. (Running on oeis4.)