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A090906
Row lengths of the irregular triangle defined in A090905.
4
1, 1, 2, 4, 6, 12, 20, 40, 80, 160, 308, 628, 1256, 2488, 5000, 9940, 19928, 39864, 79660, 159380, 318724, 637496, 1274980, 2549924, 5099884, 10199748, 20399528, 40799020, 81598052, 163196124, 326392240, 652784444, 1305568896, 2611137796
OFFSET
1,3
COMMENTS
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.
FORMULA
For n>4 a(n)= 2*(A006992(n)-A006992(n-1)) - Mohammed Bouayoun (bouyao(AT)wanadoo.fr), Mar 05 2004
MATHEMATICA
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}]; Last /@ a (* Michael De Vlieger, Dec 15 2016 *)
CROSSREFS
KEYWORD
nonn
AUTHOR
Amarnath Murthy, Dec 13 2003
EXTENSIONS
More terms from Mohammed Bouayoun (bouyao(AT)wanadoo.fr), Mar 05 2004
More terms from David Wasserman, Feb 10 2006
STATUS
approved