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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
OFFSET
1,1
COMMENTS
If all duplicates are removed the result is A036913. The indices where a(n) takes a new value are A036912. - Jeppe Stig Nielsen, Sep 28 2021
LINKS
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
KEYWORD
nonn
AUTHOR
STATUS
approved