cp's OEIS Frontend

A271649 a(n) = 2*(n^2 - n + 2).

%I A271649 #59 Nov 27 2024 15:50:05
%S A271649 4,8,16,28,44,64,88,116,148,184,224,268,316,368,424,484,548,616,688,
%T A271649 764,844,928,1016,1108,1204,1304,1408,1516,1628,1744,1864,1988,2116,
%U A271649 2248,2384,2524,2668,2816,2968,3124,3284,3448,3616,3788,3964,4144,4328,4516,4708,4904,5104,5308,5516
%N A271649 a(n) = 2*(n^2 - n + 2).
%C A271649 Numbers n such that 2*n - 7 is a perfect square.
%C A271649 Galois numbers for three-dimensional vector space, defined as the total number of subspaces in a three-dimensional vector space over GF(n-1), when n-1 is a power of a prime. - _Artur Jasinski_, Aug 31 2016, corrected by _Robert Israel_, Sep 23 2016
%H A271649 Vincenzo Librandi, <a href="/A271649/b271649.txt">Table of n, a(n) for n = 1..1000</a>
%H A271649 <a href="/index/Rec#order_03">Index entries for linear recurrences with constant coefficients</a>, signature (3,-3,1).
%F A271649 a(n) = 4*A000124(n).
%F A271649 a(n) = 2*A014206(n).
%F A271649 a(n) = A137882(n), n > 1. - _R. J. Mathar_, Apr 12 2016
%F A271649 Sum_{n>=1} 1/a(n) = tanh(sqrt(7)*Pi/2)*Pi/(2*sqrt(7)). - _Amiram Eldar_, Jul 30 2024
%F A271649 From _Elmo R. Oliveira_, Nov 18 2024: (Start)
%F A271649 G.f.: 4*x*(1 - x + x^2)/(1 - x)^3.
%F A271649 E.g.f.: 2*(exp(x)*(x^2 + 2) - 2).
%F A271649 a(n) = 3*a(n-1) - 3*a(n-2) + a(n-3) for n > 3. (End)
%e A271649 a(1) = 2*(1^2 - 1 + 2) = 4.
%p A271649 A271649:=n->2*(n^2-n+2): seq(A271649(n), n=1..60); # _Wesley Ivan Hurt_, Aug 31 2016
%t A271649 Table[2 (n^2 - n + 2), {n, 53}] (* or *)
%t A271649 Select[Range@ 5516, IntegerQ@ Sqrt[2 # - 7] &] (* or *)
%t A271649 Table[SeriesCoefficient[(-4 (1 - x + x^2))/(-1 + x)^3, {x, 0, n}], {n, 0, 52}] (* _Michael De Vlieger_, Apr 11 2016 *)
%t A271649 LinearRecurrence[{3,-3,1},{4,8,16},60] (* _Harvey P. Dale_, Jun 14 2022 *)
%o A271649 (Magma) [ 2*n^2 - 2*n + 4: n in [1..60]];
%o A271649 (Magma) [ n: n in [1..6000] | IsSquare(2*n-7)];
%o A271649 (PARI) a(n)=2*(n^2-n+2) \\ _Charles R Greathouse IV_, Jun 17 2017
%Y A271649 Cf. A000124, A014206, A137882.
%Y A271649 Numbers h such that 2*h + k is a perfect square: no sequence (k=-9), A255843 (k=-8), this sequence (k=-7), A093328 (k=-6), A097080 (k=-5), A271624 (k=-4), A051890 (k=-3), A058331 (k=-2), A001844 (k=-1), A001105 (k=0), A046092 (k=1), A056222 (k=2), A142463 (k=3), A054000 (k=4), A090288 (k=5), A268581 (k=6), A059993 (k=7), (-1)*A147973 (k=8), A139570 (k=9), A271625 (k=10), A222182 (k=11), A152811 (k=12), A181510 (k=13), A161532 (k=14), no sequence (k=15).
%K A271649 nonn,easy
%O A271649 1,1
%A A271649 _Juri-Stepan Gerasimov_, Apr 11 2016