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 A090905 #10 Dec 15 2016 10:42:16
%S A090905 1,2,3,5,9,15,27,47,87,167,327,635,1263,2519,5007,10007,19947,39875,
%T A090905 79739,159399,318779,637503,1274999,2549979,5099903,10199787,20399535,
%U A090905 40799063,81598083,163196135,326392259,652784499,1305568943,2611137839
%N A090905 Left side of irregular triangle of natural numbers in which every row product is a multiple of the previous.
%C A090905 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 A090905 The triangle goes as follows:
%e A090905 (1)
%e A090905 (2),
%e A090905 (3,4),
%e A090905 (5,6,7,8),
%e A090905 (9,10,11,12,13,14),
%e A090905 (15,16,17,18,19,20,21,22,23,24,25,26)...
%t A090905 a = {{1, 1}}; Do[k = Last@ a[[i - 1]]; While[!Divisible[Pochhammer[Total@ a[[i - 1]], k], Pochhammer @@ a[[i - 1]]], k++]; AppendTo[a, {Total@a[[i - 1]], k}], {i, 2, 17}]; a (* _Michael De Vlieger_, Dec 15 2016 *)
%Y A090905 Cf. A090904, A090906, A090907.
%K A090905 nonn
%O A090905 1,2
%A A090905 _Amarnath Murthy_, Dec 13 2003
%E A090905 More terms from _David Wasserman_, Feb 10 2006