%I #11 Oct 16 2016 09:46:23
%S 2,8,15,21,27,33,34,40,46,52,59,65,71,77,78,84,90,96,103,109,115,121,
%T 122,128,134,140,147,153,159,165,166,172,178,184,191,197,203,209,210,
%U 216,222,228,235,241,247,253,254,260,266,272,279,285,291,297,298,304
%N Numbers k such that cos(k) < 0 and cos(k+2) < 0.
%C Guide to related sequences (a four-way splitting of the natural numbers):
%C A277136: cos(k) > 0 and cos(k+2) > 0
%C A277137: cos(k) > 0 and cos(k+2) < 0
%C A277138: cos(k) < 0 and cos(k+2) > 0
%C A277139: cos(k) < 0 and cos(k+2) < 0
%H Clark Kimberling, <a href="/A277139/b277139.txt">Table of n, a(n) for n = 1..10000</a>
%t z = 400; f[x_] := Cos[x];
%t Select[Range[z], f[#] > 0 && f[# + 2] > 0 &] (* A277136 *)
%t Select[Range[z], f[#] > 0 && f[# + 2] < 0 &] (* A277137 *)
%t Select[Range[z], f[#] < 0 && f[# + 2] > 0 &] (* A277138 *)
%t Select[Range[z], f[#] < 0 && f[# + 2] < 0 &] (* A277139 *)
%o (PARI) is(n) = cos(n) < 0 && cos(n+2) < 0 \\ _Felix Fröhlich_, Oct 14 2016
%Y Cf. A277136, A277137, A277138, subsequence of A246444.
%K nonn,easy
%O 1,1
%A _Clark Kimberling_, Oct 04 2016