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 A247109 #37 Oct 11 2017 05:11:56
%S A247109 0,0,0,0,1,2,11,65,433,3271,27741,261231,2708064,30671367,377034018,
%T A247109 5001404982,71229862678,1084282429946,17571257417630,302064161086250,
%U A247109 5490937395703435,105243824522368960,2121386876912041845,44863116021267642255,993272322666679219071,22977273619066571708457
%N A247109 Number of permutations of length n with two 4-sequences.
%H A247109 Vaclav Kotesovec, <a href="/A247109/b247109.txt">Table of n, a(n) for n = 1..300</a>
%F A247109 a(n) ~ n! / n^3. - _Vaclav Kotesovec_, Nov 23 2014
%t A247109 Table[Sum[(-1)^k * k*(k-1)/2 * Sum[Sum[Binomial[k-1,p] * Binomial[k-p-1,i-2*p] * Binomial[n-2*k+p-1,n-3*k+i-1] * (n-3*k+i)!,{p,0,k-1}],{i,Max[0,3*k-n],2*(k-1)}],{k,2,n-3}],{n,1,20}] (* _Vaclav Kotesovec_, Nov 23 2014 after _Tani Akinari_ *)
%o A247109 (PARI) a(n)=sum(k=2,n-3,(-1)^k*k*(k-1)/2*sum(i=max(0,3*k-n),2*(k-1),sum(p=0,k-1,binomial(k-1,p)*binomial(k-p-1,i-2*p)*binomial(n-2*k+p-1,n-3*k+i-1)*(n-3*k+i)!)))
%Y A247109 Cf. A002630 (permutations with two 3-sequences).
%K A247109 nonn
%O A247109 1,6
%A A247109 _Tani Akinari_, Nov 21 2014