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A219395 Numbers k such that 18*k+1 is a square. 3
0, 16, 20, 68, 76, 156, 168, 280, 296, 440, 460, 636, 660, 868, 896, 1136, 1168, 1440, 1476, 1780, 1820, 2156, 2200, 2568, 2616, 3016, 3068, 3500, 3556, 4020, 4080, 4576, 4640, 5168, 5236, 5796, 5868, 6460, 6536, 7160, 7240, 7896, 7980, 8668, 8756, 9476, 9568 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

Equivalently, numbers of the form m*(18*m+2), where m = 0,-1,1,-2,2,-3,3,...

Also, integer values of 2*h*(h+1)/9.

LINKS

Bruno Berselli, Table of n, a(n) for n = 1..1000

Index entries for linear recurrences with constant coefficients, signature (1,2,-2,-1,1).

FORMULA

G.f.: 4*x^2*(4 + x + 4*x^2)/((1 + x)^2*(1 - x)^3).

a(n) = a(-n+1) = (18*n*(n-1) + 7*(-1)^n*(2*n-1) - 1)/4 + 2.

Sum_{n>=2} 1/a(n) = 9/2 - cot(Pi/9)*Pi/2. - Amiram Eldar, Mar 15 2022

MAPLE

A219395:=proc(q)

local n;

for n from 1 to q do if type(sqrt(18*n+1), integer) then print(n);

fi; od; end:

A219395(1000); # Paolo P. Lava, Feb 19 2013

MATHEMATICA

Select[Range[0, 10000], IntegerQ[Sqrt[18 # + 1]] &]

CoefficientList[Series[4 x (4 + x + 4 x^2)/((1 + x)^2 (1 - x)^3), {x, 0, 50}], x] (* Vincenzo Librandi, Aug 18 2013 *)

LinearRecurrence[{1, 2, -2, -1, 1}, {0, 16, 20, 68, 76}, 50] (* Harvey P. Dale, Dec 24 2014 *)

PROG

(Magma) [n: n in [0..10000] | IsSquare(18*n+1)];

(Magma) I:=[0, 16, 20, 68, 76]; [n le 5 select I[n] else Self(n-1)+2*Self(n-2)-2*Self(n-3)-Self(n-4)+Self(n-5): n in [1..50]]; // Vincenzo Librandi, Aug 18 2013

CROSSREFS

Cf. similar sequences listed in A219257.

Sequence in context: A102544 A152022 A213241 * A260572 A240038 A188242

Adjacent sequences: A219392 A219393 A219394 * A219396 A219397 A219398

KEYWORD

nonn,easy

AUTHOR

Bruno Berselli, Dec 03 2012

STATUS

approved

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Last modified December 5 09:35 EST 2022. Contains 358585 sequences. (Running on oeis4.)