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A378641 Largest m <= n such that phi(m) does not divide n, or -1 if no such m exists, where phi is the Euler totient function (A000010).

%I A378641 #10 Dec 06 2024 11:32:11
%S A378641 -1,-1,3,-1,5,5,7,7,9,10,11,11,13,14,15,14,17,17,19,20,21,22,23,23,25,
%T A378641 26,27,28,29,30,31,31,33,34,35,35,37,38,39,40,41,42,43,44,45,46,47,47,
%U A378641 49,50,51,52,53,53,55,56,57,58,59,60,61,62,63,63,65,66,67,68,69
%N A378641 Largest m <= n such that phi(m) does not divide n, or -1 if no such m exists, where phi is the Euler totient function (A000010).
%H A378641 Paolo Xausa, <a href="/A378641/b378641.txt">Table of n, a(n) for n = 1..10000</a>
%F A378641 a(n) = n if n is an odd number >= 3.
%t A378641 A378641[n_] := Module[{m = n}, While[m > 0 && Divisible[n, EulerPhi[m]], m--]; If[m == 0, -1, m]];
%t A378641 Array[A378641, 100]
%Y A378641 Cf. A000010, A378636, A378637, A378640, A378642.
%K A378641 sign,easy
%O A378641 1,3
%A A378641 _Paolo Xausa_, Dec 05 2024