A249055 a(1)=0; the next term is always the product of the two smallest numbers not yet in the sequence and which have not yet been used.

0, 2, 12, 30, 56, 90, 143, 210, 272, 342, 420, 506, 600, 702, 812, 992, 1122, 1260, 1406, 1560, 1722, 1892, 2070, 2256, 2450, 2652, 2862, 3135, 3422, 3660, 3906, 4160, 4422, 4692, 4970, 5256, 5550, 5852, 6162, 6480, 6806, 7140, 7482, 7832, 8372, 8742, 9120, 9506, 9900

Offset: 1

Author

N. J. A. Sloane, Nov 01 2014

Keywords

• nonn

Comments

Suggested by A075336 and A249406.

Examples

			Start with a(1) = 0. The missing numbers are 1 2 3 4 5 6 ...
Multiply the first two, and we get 2, which is therefore a(2).
Cross 1, 2, and 1*2 = 2 off the missing list.
The first two missing numbers are now 3 and 4, so a(3) = 3*4 = 12.
Cross off 3,4,12 from the missing list.
Repeat!
		

Crossrefs

Cf. A075326, A000045, A249406, A000304, A000301.

Programs

Maple

M:=50; A:=[0]; miss:=[seq(n,n=1..M^2)]:
for n from 1 to M do t1:=miss[1]*miss[2]; A:=[op(A),t1];
miss:=[seq(miss[i],i=3..nops(miss))];
miss:=remove('x->x=t1',miss);
od:
A;

Extensions

Typo in definition corrected by Douglas Latimer, Nov 01 2014