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A161320
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Floor of sum_{k=1..n} sum_{m=1..k} ( 2* cosh(pi*m/(2*k))-1 ).
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0
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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
(list; graph; refs; listen; history; internal format)
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OFFSET
| 1,1
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COMMENTS
| Pi in the definition is A000796.
a(n) appears to oscillate near (n+1)^2.
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FORMULA
| a(n) = floor of ( 2* sum_{k=1..n} sum_{m=1..k} cosh(pi*m/(2k)) - A000217(n) ).
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MATHEMATICA
| Floor[N[Table[Sum[(2*Cos[Pi*I*i/(2j)]-1), {j, 1, k}, {i, 1, j}], {k, 1, 20}]]]
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CROSSREFS
| Sequence in context: A073173 A092464 A173562 * A170879 A134578 A189979
Adjacent sequences: A161317 A161318 A161319 * A161321 A161322 A161323
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KEYWORD
| nonn
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AUTHOR
| Daniel Tisdale (daniel6874(AT)gmail.com), Jun 06 2009
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EXTENSIONS
| Variables redefined, sequence extended - R. J. Mathar (mathar(AT)strw.leidenuniv.nl), Aug 30 2010
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