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 A055142 #20 Aug 30 2023 11:42:44
%S A055142 1,0,-2,-8,-36,-224,-1880,-19872,-251888,-3712256,-62286624,
%T A055142 -1171487360,-24402416192,-557542291968,-13861636770176,
%U A055142 -372514645389824,-10759590258589440,-332386419622387712
%N A055142 Expansion of e.g.f.: exp(x)*sqrt(1-2x).
%H A055142 Vincenzo Librandi, <a href="/A055142/b055142.txt">Table of n, a(n) for n = 0..200</a>
%F A055142 a(n) ~ -2^(n-1/2) * n^(n-1) / exp(n-1/2). - _Vaclav Kotesovec_, Mar 29 2014
%F A055142 D-finite with recurrence: a(n) +2*(-n+1)*a(n-1) +2*(n-1)*a(n-2)=0. - _R. J. Mathar_, Jan 22 2020
%t A055142 CoefficientList[Series[E^x*Sqrt[1-2*x], {x, 0, 20}], x] * Range[0, 20]! (* _Vaclav Kotesovec_, Mar 29 2014 *)
%o A055142 (Sage)
%o A055142 def A055142(n):
%o A055142 @CachedFunction
%o A055142 def h(n):
%o A055142 return (-1)^n*2*(n-2)*abs(h(n-1)+h(n-2)) if n>1 else 1
%o A055142 return -(-1)^(n+1)*h(n+1)
%o A055142 [A055142(n) for n in (0..17)] # _Peter Luschny_, Nov 02 2012
%Y A055142 Cf. A001147, A053871.
%Y A055142 Column 0 of triangle A055141.
%K A055142 sign
%O A055142 0,3
%A A055142 _Christian G. Bower_, May 09 2000