A090906 Row lengths of the irregular triangle defined in A090905.

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.

Links

Table of n, a(n) for n=1..34.

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

Cf. A090904, A090905, A090907.

Sequence in context: A164146 A370582 A279245 * A047141 A341858 A105541

Adjacent sequences: A090903 A090904 A090905 * A090907 A090908 A090909

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