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 A216206 #24 Aug 10 2025 02:37:58
%S A216206 1,-3,-9,81,1215,-40095,-2525985,325852065,83092276575,
%T A216206 -42626337882975,-43606743654283425,89350217747626737825,
%U A216206 365889141676531491393375,-2997729737755822508985921375,-49111806293653640164716349886625,1609344780436736134557590069434814625
%N A216206 a(n) = Product_{i=1..n} ((-2)^i-1).
%C A216206 Signed partial products of A062510. This implies that all terms from a(1) on are multiples of 3.
%F A216206 A015109(n,k) = a(n)/(a(k)*a(n-k)).
%F A216206 a(n) = (-3)^n*A015013(n) for n>0, a(0)=1. - _Bruno Berselli_ and _Alonso del Arte_, Mar 13 2013
%F A216206 a(n) ~ (-1)^(floor(n/2)+1) * c * 2^(n*(n+1)/2), where c = Product_{k>=1} (1 - 1/(-2)^k) = 1.21072413030105918013... (A330862). - _Amiram Eldar_, Aug 10 2025
%p A216206 A216206 := proc(n)
%p A216206 mul( (-2)^i-1, i=1..n) ;
%p A216206 end proc:
%t A216206 Table[(-1)^n QPochhammer[-2, -2, n], {n, 0, 15}] (* _Bruno Berselli_, Mar 13 2013 *)
%t A216206 Table[Product[(-2)^k-1,{k,n}],{n,0,20}] (* _Harvey P. Dale_, Oct 21 2024 *)
%Y A216206 Cf. A005329, A015013, A015109, A027871, A062510, A330862.
%K A216206 sign,easy
%O A216206 0,2
%A A216206 _R. J. Mathar_, Mar 12 2013