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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 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS Pi in the definition is A000796. a(n) appears to oscillate near (n+1)^2. LINKS 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: A092464 A328271 A173562 * A170879 A134578 A189979 Adjacent sequences:  A161317 A161318 A161319 * A161321 A161322 A161323 KEYWORD nonn AUTHOR Daniel Tisdale, Jun 06 2009 EXTENSIONS Variables redefined, sequence extended by R. J. Mathar, Aug 30 2010 STATUS approved

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Last modified July 31 08:59 EDT 2021. Contains 346369 sequences. (Running on oeis4.)