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

Author

Reinhard Zumkeller, Oct 31 2014

Keywords

• nonn

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

Crossrefs

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

Programs

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