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 A259211 Positive octagonal numbers (A000567) that are other octagonal numbers divided by 3. 2
 560, 4090071480, 29860802042948800, 218007803276760596488520, 1591631806178550455562538466640, 11620188673811755515268615201727379160, 84836696710140620657577957851546090689242080, 619375925014776443389412658082968133380998230391400 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 LINKS Colin Barker, Table of n, a(n) for n = 1..146 Index entries for linear recurrences with constant coefficients, signature (7300803,-7300803,1). FORMULA a(n) = 7300803*a(n-1)-7300803*a(n-2)+a(n-3). G.f.: -40*x*(x^2+40545*x+14) / ((x-1)*(x^2-7300802*x+1)). EXAMPLE 560 is in the sequence because 560 is the 14th octagonal number, and 3*560=1680 is the 24th octagonal number. MATHEMATICA CoefficientList[Series[40 (x^2 + 40545 x + 14)/((1 - x) (x^2 - 7300802 x + 1)), {x, 0, 40}], x] (* Vincenzo Librandi, Jun 21 2015 *) PROG (PARI) Vec(-40*x*(x^2+40545*x+14)/((x-1)*(x^2-7300802*x+1)) + O(x^20)) CROSSREFS Cf. A000567, A076139, A121338, A259210. Sequence in context: A229389 A167822 A104590 * A265261 A234936 A274443 Adjacent sequences:  A259208 A259209 A259210 * A259212 A259213 A259214 KEYWORD nonn,easy AUTHOR Colin Barker, Jun 21 2015 STATUS approved

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Last modified September 21 00:14 EDT 2021. Contains 347596 sequences. (Running on oeis4.)