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 A090907 #8 Dec 15 2016 10:39:23 %S A090907 2,6,140,1287,2139552000,2949442889323392, %T A090907 322686644032484531917367528014184448000000 %N A090907 Ratio of products of successive rows of the irregular triangle defined in A090905. %C A090907 Conjecture: For n > 4 the last term of the n-th group is 2p where p is the largest prime in the (n-1)th group. And these are the Bertrand primes. %e A090907 a(1)=(2!/1!)*(0!/1!) %e A090907 a(2)=(4!/2!)*(1!/2!) %e A090907 a(3)=(8!/4!)*(2!/4!) %e A090907 a(4)=(14!/8!)*(4!/8!) %e A090907 a(5)=(26!/14!)*(8!/14!) %e A090907 a(6)=(46!/26!)*(14!/26!) %e A090907 For n>=6 we have a(n)= ((2*A006992(n))!/(2*A006992(n-1))!)*((2*A006992(n-2))!/(2*A006992(n-1))!), verified for 4<n<21 %Y A090907 Cf. A090904, A090905, A090906. %K A090907 nonn %O A090907 1,1 %A A090907 _Amarnath Murthy_, Dec 13 2003 %E A090907 Edited by Mohammed Bouayoun (bouyao(AT)wanadoo.fr), Mar 05 2004