This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A178075 #22 Jan 05 2025 19:51:39
%S A178075 0,1,1,-2,1,9,-16,-73,-145,1442,3951,-49121,273184,4095073,-13134671,
%T A178075 -609740514,-7877081935,206841213449,153991537616,-138343392420553,
%U A178075 3626684171795871,187758843981508546,2146096698964614559
%N A178075 A (1,2) Somos-4 sequence.
%C A178075 Hankel transform of A178074 is a(n+2).
%C A178075 This is a strong elliptic divisibility sequence t_n as given in [Kimberling, p. 16] where x = 1, y = -2, z = 1. - _Michael Somos_, Aug 06 2014
%H A178075 G. C. Greubel, <a href="/A178075/b178075.txt">Table of n, a(n) for n = 0..150</a>
%H A178075 Paul Barry, <a href="https://arxiv.org/abs/1807.05794">Riordan Pseudo-Involutions, Continued Fractions and Somos 4 Sequences</a>, arXiv:1807.05794 [math.CO], 2018.
%H A178075 C. Kimberling, <a href="https://web.archive.org/web/2024*/https://www.fq.math.ca/Scanned/17-1/kimberling1.pdf">Strong divisibility sequences and some conjectures</a>, Fib. Quart., 17 (1979), 13-17.
%F A178075 a(n) = (a(n-1)*a(n-3) + 2*a(n-2)^2)/a(n-4), n>=4.
%F A178075 a(-n) = -a(n). - _Michael Somos_, Aug 06 2014
%t A178075 Join[{0}, RecurrenceTable[{a[n] == (a[n - 1]*a[n - 3] + 2*a[n - 2]^2)/a[n - 4], a[1] == 1, a[2] == 1, a[3] == -2, a[4] == 1}, a, {n, 1, 50}]] (* _G. C. Greubel_, Sep 22 2018 *)
%o A178075 (Magma) I:=[0,1,1,-2,1]; [n le 5 select I[n] else (Self(n-1)*Self(n-3)+2*Self(n-2)^2)/Self(n-4): n in [1..30]]; // _Vincenzo Librandi_, Aug 07 2014
%o A178075 (PARI) m=50; v=concat([1,1,-2,1], vector(m-4)); for(n=5, m, v[n] = (v[n-1]*v[n-3] + 2*v[n-2]^2)/v[n-4]); concat([0], v) \\ _G. C. Greubel_, Sep 22 2018
%K A178075 easy,sign
%O A178075 0,4
%A A178075 _Paul Barry_, May 19 2010
%E A178075 Added missing a(0)=0 and a(1)=1. - _Michael Somos_, Aug 06 2014