A249406 Start with a(1) = 1, and extend by the rule that the next term is the product of the two most recent non-terms of the sequence.

1, 6, 20, 56, 90, 132, 182, 240, 306, 399, 506, 600, 702, 812, 930, 1056, 1190, 1332, 1482, 1640, 1806, 1980, 2162, 2352, 2550, 2756, 2970, 3306, 3540, 3782, 4032, 4290, 4556, 4830, 5112, 5402, 5700, 6006, 6320, 6642, 6972, 7310, 7656, 8099, 8556, 8930, 9312

Offset

1,2

Comments

Compare to A075326, where not products, but sums of the two most recent non-terms are considered;

a(195) = 159200 is the smallest even term not of the form m*(m+1); see also A249408, the set of all non-oblong terms of this sequence.

a(10) = 399 is the smallest odd term.

Links

Reinhard Zumkeller, Table of n, a(n) for n = 1..10000

Prog

(Haskell)
import Data.List ((\\))
a249406 n = a249406_list !! (n-1)
a249406_list = 1 : f [2..] where
   f ws@(u:v:_) = y : f (ws \\ [u, v, y]) where y = u * v

Crossrefs

Cf. A000301, A000304, A075326, A249055, A249407 (complement), subsequence of A002808.

Sequence in context: A328681 A109903 A323640 * A220020 A201149 A260777

Adjacent sequences: A249403 A249404 A249405 * A249407 A249408 A249409

Keyword

nonn

Author

Reinhard Zumkeller, Oct 31 2014

Status

approved