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URL: https://oeis.org/A277137

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A277137
Numbers k such that cos(k) > 0 and cos(k+2) < 0.
4
1, 6, 7, 13, 14, 19, 20, 25, 26, 31, 32, 38, 39, 44, 45, 50, 51, 57, 58, 63, 64, 69, 70, 75, 76, 82, 83, 88, 89, 94, 95, 101, 102, 107, 108, 113, 114, 119, 120, 126, 127, 132, 133, 138, 139, 145, 146, 151, 152, 157, 158, 163, 164, 170, 171, 176, 177, 182
OFFSET
1,2
COMMENTS
Guide to related sequences (a four-way splitting of the natural numbers):
A277136: cos(k) > 0 and cos(k+2) > 0
A277137: cos(k) > 0 and cos(k+2) < 0
A277138: cos(k) < 0 and cos(k+2) > 0
A277139: cos(k) < 0 and cos(k+2) < 0
LINKS
MATHEMATICA
z = 400; f[x_] := Cos[x];
Select[Range[z], f[#] > 0 && f[# + 2] > 0 &] (* A277136 *)
Select[Range[z], f[#] > 0 && f[# + 2] < 0 &] (* A277137 *)
Select[Range[z], f[#] < 0 && f[# + 2] > 0 &] (* A277138 *)
Select[Range[z], f[#] < 0 && f[# + 2] < 0 &] (* A277139 *)
Position[Partition[Table[Cos[n], {n, 200}], 3, 1], _?(#[[1]]>0&&#[[3]]<0&), 1, Heads->False]//Flatten (* Harvey P. Dale, Jan 26 2018 *)
PROG
(PARI) is(n) = cos(n) > 0 && cos(n+2) < 0 \\ Felix Fröhlich, Oct 14 2016
CROSSREFS
Cf. A277136, A277138, A277139, subsequence of A131503.
Sequence in context: A069136 A348368 A389738 * A047335 A182623 A127020
KEYWORD
nonn,easy
AUTHOR
Clark Kimberling, Oct 04 2016
STATUS
approved