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 A259156 Positive triangular numbers (A000217) that are pentagonal numbers (A000326) divided by 2. 12
 6, 105, 58311, 1008910, 559902916, 9687554415, 5376187741821, 93019896484620, 51622154137063026, 893177036357767525, 495675918647891434531, 8576285810087387291130, 4759480119234899417304336, 82349495455282056411663435, 45700527609217585557064800441 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS Intersection of A000217 and A193866 (even pentagonal numbers divided by 2). - Michel Marcus, Jun 20 2015 LINKS Colin Barker, Table of n, a(n) for n = 1..502 Index entries for linear recurrences with constant coefficients, signature (1,9602,-9602,-1,1). FORMULA G.f.: -x*(x^3+594*x^2+99*x+6) / ((x-1)*(x^2-98*x+1)*(x^2+98*x+1)). EXAMPLE 6 is in the sequence because 6 is the 3rd triangular number, and 2*6 is the 3rd pentagonal number. MATHEMATICA LinearRecurrence[{1, 9602, -9602, -1, 1}, {6, 105, 58311, 1008910, 559902916}, 20] (* Vincenzo Librandi, Jun 20 2015 *) PROG (PARI) Vec(-x*(x^3+594*x^2+99*x+6)/((x-1)*(x^2-98*x+1)*(x^2+98*x+1)) + O(x^20)) CROSSREFS Cf. A000217, A000326, A193866, A259157-A259167. Sequence in context: A109819 A162130 A048707 * A075068 A055763 A213464 Adjacent sequences:  A259153 A259154 A259155 * A259157 A259158 A259159 KEYWORD nonn,easy AUTHOR Colin Barker, Jun 19 2015 STATUS approved

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Last modified August 12 10:31 EDT 2020. Contains 336438 sequences. (Running on oeis4.)