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%I #53 Oct 23 2020 03:28:38
%S 1,2,4,5,8,10,11,13,16,17,19,20,22,23,25,26,29,31,32,34,37,38,40,41,
%T 43,44,46,47,50,52,53,55,58,59,61,62,64,65,67,68,71,73,74,76,79,80,82,
%U 83,85,86,88,89,92,94,95,97,100,101,103,104,106,107,109,110,113,115,116
%N Numbers coprime to 21.
%C The asymptotic density of this sequence is 4/7. - _Amiram Eldar_, Oct 23 2020
%H Amiram Eldar, <a href="/A160545/b160545.txt">Table of n, a(n) for n = 1..10000</a>
%H <a href="/index/Rec#order_13">Index entries for linear recurrences with constant coefficients</a>, signature (1,0,0,0,0,0,0,0,0,0,0,1,-1).
%F a(n+12) = a(n) + 21.
%F a(n) = 21*floor((n-1)/12)+f(n)+floor(f(n)/2)+2^floor(f(n)/4)+floor(((n+5) mod 12)/11)+floor(((n+3) mod 12)/11), where f(n)= (n-1) mod 12. - _Gary Detlefs_, Sep 22 2013
%F G.f.: x*(1+x+2*x^2+x^3+3*x^4+2*x^5+x^6+2*x^7+3*x^8+x^9+2*x^10+x^11+x^12) / ( (1+x) *(1+x^2) *(1+x+x^2) *(x^2-x+1) *(x^4-x^2+1) *(x-1)^2 ). - _R. J. Mathar_, Sep 27 2014
%t LinearRecurrence[{1,0,0,0,0,0,0,0,0,0,0,1,-1}, 1,2,4,5,8,10,11,13,16,17,19,20,22},67] (* _Ray Chandler_, Jul 15 2015 *)
%t Select[Range[100], CoprimeQ[21, #] &] (* _Amiram Eldar_, Oct 23 2020 *)
%o (Sage) [i for i in range(0,128) if gcd(21, i) == 1]
%Y Cf. A162699, A045572.
%K nonn,easy
%O 1,2
%A _Zerinvary Lajos_, May 18 2009
%E Definition corrected by _Leroy Quet_, Jun 19 2009
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