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A251624
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Numbers n such that the octagonal numbers N(n), N(n+1) and N(n+2) sum to another octagonal number.
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2
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278, 752958, 2034494038, 5497202139518, 14853438146485398, 40133984374601407678, 108442010926734857062358, 293010273390053209181085438, 791713650257912844472435792918, 2139209989986607115711312331380798, 5780144601230162168739121446955125078
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OFFSET
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1,1
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COMMENTS
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Also nonnegative integers x in the solutions to 18*x^2-6*y^2+24*x+4*y+18 = 0, the corresponding values of y being A251625.
It seems that the least significant digit of each term is 8.
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LINKS
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FORMULA
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a(n) = 2703*a(n-1)-2703*a(n-2)+a(n-3).
G.f.: 2*x*(x^2-762*x-139) / ((x-1)*(x^2-2702*x+1)).
a(n) = 2*(-3 - (2*sqrt(3)+3)*(1351+780*sqrt(3))^(-n) + (2*sqrt(3)-3)*(1351+780*sqrt(3))^n) / 9. - Colin Barker, May 30 2017
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EXAMPLE
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278 is in the sequence because N(278)+N(279)+N(280) = 231296+232965+234640 = 698901 = N(483).
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MATHEMATICA
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LinearRecurrence[{2703, -2703, 1}, {278, 752958, 2034494038}, 20] (* Harvey P. Dale, Feb 14 2015 *)
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PROG
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(PARI) Vec(2*x*(x^2-762*x-139)/((x-1)*(x^2-2702*x+1)) + O(x^100))
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CROSSREFS
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KEYWORD
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nonn,easy
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AUTHOR
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STATUS
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approved
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