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A161320
Floor of Sum_{k=1..n} Sum_{m=1..k} (2*cosh(Pi*m/(2*k)) - 1).
0
4, 9, 17, 26, 37, 51, 66, 83, 102, 122, 145, 170, 197, 225, 256, 288, 323, 359, 397, 437, 479, 523, 569, 617, 667, 718, 772, 828, 885, 945, 1006, 1069, 1135, 1202, 1271, 1342, 1415, 1490, 1566, 1645, 1726, 1808, 1893, 1979, 2068, 2158, 2250, 2344, 2440, 2539
OFFSET
1,1
COMMENTS
Pi in the definition is A000796.
a(n) appears to oscillate near (n+1)^2.
FORMULA
a(n) = floor(2*Sum_{k=1..n} Sum_{m=1..k} cosh(Pi*m/(2*k)) - A000217(n)).
MATHEMATICA
Floor[N[Table[Sum[(2*Cos[Pi*I*i/(2j)]-1), {j, 1, k}, {i, 1, j}], {k, 1, 20}]]]
CROSSREFS
Sequence in context: A328271 A354414 A173562 * A170879 A134578 A189979
KEYWORD
nonn
AUTHOR
Daniel Tisdale, Jun 06 2009
EXTENSIONS
Variables redefined, sequence extended by R. J. Mathar, Aug 30 2010
STATUS
approved