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 A323425 #11 Jan 16 2019 17:39:15
%S A323425 1,0,0,0,9,648,57600,6615000,972504225,179499220992,40789783609344,
%T A323425 11212877910528000,3671848787797265625,1413385410212064432000,
%U A323425 632129969391038455873536,325176984737061807515098752,190691488202627199302740850625,126479088749202444199526400000000
%N A323425 Number of ways n people in a line can each choose two others both on the same side of them.
%F A323425 a(n) = Product_{k = 1..n} ( binomial(k-1, 2) + binomial(n-k, 2) ).
%F A323425 a(n) ~ exp(Pi*(n/2 - 1) - 2*n) * n^(2*n) / 2^n. - _Vaclav Kotesovec_, Jan 15 2019
%e A323425 Example: For n = 4, with four people ABCD, A can choose any two of {B, C, D} (3 choices), B can choose {C, D} (1 choice), C can choose {A, B} (1 choice), and D can choose any two of {A, B, C} (3 choices), so there are 3*1*1*3=9 possible overall choices and a(4) = 9.
%e A323425 Example: For n = 5, with five people ABCDE, A can choose any two of {B, C, D, E} (6 choices), B can choose any two of {C, D, E} (3 choices), C can choose either {A, B} or {D, E} (2 choices), D can choose any two of {A, B, C} (3 choices), E can choose any two of {A, B, C, D} (6 choices), so a(5) = 6*3*2*3*6 = 648.
%t A323425 Table[Product[Binomial[k-1, 2] + Binomial[n-k, 2], {k, 1, n}], {n, 0, 20}] (* _Vaclav Kotesovec_, Jan 15 2019 *)
%o A323425 (Haskell) a n = product [(k-1)*(k-2) `div` 2 + (n-k)*(n-k-1) `div` 2 | k<-[1..n]]
%K A323425 nonn
%O A323425 0,5
%A A323425 _Shreevatsa R_, Jan 14 2019
%E A323425 More terms from _Vaclav Kotesovec_, Jan 15 2019