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A209211 Numbers n such that n-1 and phi(n) are relatively prime.

%I A209211 #36 Dec 30 2016 06:07:16
%S A209211 1,2,4,6,8,10,12,14,16,18,20,22,24,26,30,32,34,36,38,40,42,44,46,48,
%T A209211 50,54,56,58,60,62,64,68,72,74,78,80,82,84,86,88,90,92,94,96,98,100,
%U A209211 102,104,106,108,110,114,116,118,120,122,126,128,132,134,136,138
%N A209211 Numbers n such that n-1 and phi(n) are relatively prime.
%C A209211 A063994(a(n)) = 1. - _Reinhard Zumkeller_, Mar 02 2013
%C A209211 a(n) = A111305(n-2) for n >= 3. - _Emmanuel Vantieghem_, Jul 03 2013
%C A209211 n such that A049559(n) = 1.  Includes A100484 and A000079. - _Robert Israel_, Nov 09 2015
%C A209211 All terms except the first one are even. Missing even terms are A039772. - _Robert G. Wilson v_, Sep 26 2016
%C A209211 Numbers n such that A187730(n) = 1. - _Thomas Ordowski_, Dec 29 2016
%H A209211 Reinhard Zumkeller, <a href="/A209211/b209211.txt">Table of n, a(n) for n = 1..10000</a>
%p A209211 select(n -> igcd(n-1, numtheory:-phi(n)) = 1, [$1..1000]); # _Robert Israel_, Nov 09 2015
%t A209211 Select[Range[200], GCD[# - 1, EulerPhi[#]] == 1 &]
%o A209211 (Haskell)
%o A209211 a209211 n = a209211_list !! (n-1)
%o A209211 a209211_list = filter (\x -> (x - 1) `gcd` a000010 x == 1) [1..]
%o A209211 -- _Reinhard Zumkeller_, Mar 02 2013
%o A209211 (PARI) isok(n) = gcd(n-1, eulerphi(n)) == 1; \\ _Michel Marcus_, Sep 26 2016
%Y A209211 Cf. A000010, A000079, A003277, A039772, A049559, A063994, A100484, A111305.
%K A209211 nonn
%O A209211 1,2
%A A209211 _José María Grau Ribas_, Mar 06 2012