OFFSET
1,2
COMMENTS
Equivalently, numbers of the form m*(22*m+2), where m = 0,-1,1,-2,2,-3,3,...
Also, integer values of 2*h*(h+1)/11.
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*(5 + x + 5*x^2)/((1 + x)^2*(1 - x)^3).
a(n) = a(-n+1) = (22*n*(n-1) + 9*(-1)^n*(2*n - 1) + 1)/4 + 2.
Sum_{n>=2} 1/a(n) = 11/2 - cot(Pi/11)*Pi/2. - Amiram Eldar, Mar 16 2022
MAPLE
A219392:=proc(q)
local n;
for n from 1 to q do if type(sqrt(22*n+1), integer) then print(n);
fi; od; end:
A219392(1000); # Paolo P. Lava, Feb 19 2013
MATHEMATICA
Select[Range[0, 11000], IntegerQ[Sqrt[22 # + 1]] &]
CoefficientList[Series[4 x (5 + x + 5 x^2)/((1 + x)^2 (1 - x)^3), {x, 0, 50}], x] (* Vincenzo Librandi, Aug 18 2013 *)
PROG
(Magma) [n: n in [0..11000] | IsSquare(22*n+1)];
(Magma) I:=[0, 20, 24, 84, 92]; [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
KEYWORD
nonn,easy
AUTHOR
Bruno Berselli, Nov 22 2012
STATUS
approved