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A077400
Triangular numbers that are 7 times triangular numbers.
11
0, 21, 105, 5460, 26796, 1386945, 6806205, 352278696, 1728749400, 89477401965, 439095541521, 22726907820540, 111528538797060, 5772545109015321, 28327809758911845, 1466203730782071120, 7195152150224811696, 372409975073537049285, 1827540318347343259065
OFFSET
0,2
FORMULA
Let b(n) = A077399(n) then a(n) = 7*b(n).
G.f.: -21*x*(x^2+4*x+1) / ((x-1)*(x^2-16*x+1)*(x^2+16*x+1)). - Colin Barker, Jul 02 2013
From _Vladimir Pletser, Feb 21 2021: (Start)
a(n) = 254*a(n - 2) - a (n - 4) + 126.
a(n) = a(n - 1) + 254*(a(n - 2) - a(n - 3)) - (a (n - 4) - a(n - 5)). (End)
MAPLE
f := gfun:-rectoproc({a(-2) = 21, a(-1) = 0, a(0) = 0, a(1) = 21, a(n) = 254*a(n-2)-a(n-4)+126}, a(n), remember): map(f, [`$`(0 .. 1000)])[] # Vladimir Pletser, Feb 21 2021
MATHEMATICA
LinearRecurrence[{1, 254, -254, -1, 1}, {0, 21, 105, 5460, 26796}, 20] (* Harvey P. Dale, Oct 28 2013 *)
PROG
(PARI) concat(0, Vec(-21*x*(x^2+4*x+1)/((x-1)*(x^2-16*x+1)*(x^2+16*x+1)) + O(x^100))) \\ Colin Barker, May 15 2015
(Magma) Q:=Rationals(); R<x>:=PowerSeriesRing(Q, 30); [0] cat Coefficients(R!(-21*x*(x^2+4*x+1) / ((x-1)*(x^2-16*x+1)*(x^2+16*x+1)))); // G. C. Greubel, Jan 18 2018
CROSSREFS
KEYWORD
easy,nonn
AUTHOR
Bruce Corrigan (scentman(AT)myfamily.com), Nov 05 2002
STATUS
approved