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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
OFFSET
1,1
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
KEYWORD
nonn,easy
AUTHOR
Colin Barker, Jun 21 2015
STATUS
approved