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A156712 Star numbers (A003154) that are also triangular numbers (A000217). 2
1, 7, 91, 1261, 17557, 244531, 3405871, 47437657, 660721321, 9202660831, 128176530307, 1785268763461, 24865586158141, 346332937450507, 4823795538148951, 67186804596634801, 935791468814738257, 13033893758809700791, 181538721154521072811 (list; graph; refs; listen; history; text; internal format)
OFFSET
1,2
COMMENTS
From Colin Barker, Jan 06 2015: (Start)
Also indices of centered square numbers (A001844) which are also centered triangular numbers (A005448).
Also indices of centered octagonal numbers (A016754) which are also centered hexagonal numbers (A003215).
Also positive integers y in the solutions to 3*x^2-4*y^2-3*x+4*y = 0, the corresponding values of x being A001922.
(End)
LINKS
Giovanni Lucca, Circle Chains Inscribed in Symmetrical Lenses and Integer Sequences, Forum Geometricorum, Volume 16 (2016) 419-427.
Wikipedia, Star Numbers
FORMULA
a(n+3) = 15*a(n+2) - 15*a(n+1) + a(n).
If x^2 - 3*y^2 = 1 with x even then a(y) = (y+2)/4 evidently related to A001570 by: add 1 and halve.
G.f.: x*(1 - 8*x + x^2)/((1-x)*(1 - 14*x + x^2)). - Alexander R. Povolotsky, Feb 15 2009
a(n) = (4 + (2 + sqrt(3))*(7 - 4*sqrt(3))^n + (2 - sqrt(3))*(7 + 4*sqrt(3))^n)/8. - Colin Barker, Mar 05 2016
a(n) = (1/2)*( 1 + ChebyshevU(n, 7) - 13*ChebyshevU(n-1, 7) ). - G. C. Greubel, Oct 07 2022
MAPLE
f:= gfun[rectoproc]({a(n+3)=15*a(n+2)-15*a(n+1)+a(n), a(1)=1, a(2)=7, a(3)=91}, a(n), 'remember'):
seq(f(n), n=1..30); # Robert Israel, Jan 01 2015
MATHEMATICA
f[n_] := (Simplify[(2 + Sqrt@3)^(2 n - 1) + (2 - Sqrt@3)^(2 n - 1)] + 4)/8; Array[f, 17] (* Robert G. Wilson v, Oct 28 2010 *)
PROG
(PARI) Vec(-x*(x^2-8*x+1)/((x-1)*(x^2-14*x+1)) + O(x^100)) \\ Colin Barker, Jan 01 2015
(Magma) [(Evaluate(ChebyshevSecond(n+1), 7) - 13*Evaluate(ChebyshevU(n), 7) + 1)/2: n in [1..30]]; // G. C. Greubel, Oct 07 2022
(SageMath)
def A156712(n): return (1 + chebyshev_U(n, 7) - 13*chebyshev_U(n-1, 7))/2
[A156712(n) for n in range(1, 31)] # G. C. Greubel, Oct 07 2022
CROSSREFS
Sequence in context: A248226 A165230 A346939 * A004368 A243946 A130978
KEYWORD
easy,nonn
AUTHOR
Aaron Meyerowitz, Feb 14 2009
EXTENSIONS
a(11) onwards from Robert G. Wilson v, Oct 28 2010
STATUS
approved

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Last modified June 25 21:12 EDT 2024. Contains 373712 sequences. (Running on oeis4.)