A199162 a(1) = 1, a(2) = 6; for n>2, a(n) = least number > a(n-1) which is a unique sum of two distinct earlier terms.

1, 6, 7, 8, 9, 10, 11, 12, 14, 24, 26, 27, 28, 29, 31, 45, 46, 47, 48, 49, 62, 68, 82, 83, 84, 85, 98, 104, 117, 122, 135, 142, 154, 155, 159, 172, 191, 192, 193, 194, 195, 209, 234, 245, 248, 249, 250, 265, 266, 267, 268, 270, 283, 302, 303, 304, 305, 306

Offset: 1

Author

Reinhard Zumkeller, Nov 03 2011

Keywords

• nonn

Comments

An Ulam-type sequence - see A002858 for many further references, comments, etc.

Examples

			13 is not a term, as 13 = 7+6 = a(3)+a(2) and 13 = 12+1 = a(8)+a(1);
14 is a term, because 14 = 8 + 6 = a(4) + a(2) is unique for distinct terms, a(9) = 14.
		

Links

• Reinhard Zumkeller, Table of n, a(n) for n = 1..10000
• Eric Weisstein's World of Mathematics, Ulam Sequence
• Wikipedia, Ulam number
• Index entries for Ulam numbers

Crossrefs

Cf. A003663.

Programs

Haskell

a199162 n = a199162_list !! (n-1)
a199162_list = 1 : 6 : ulam 2 6 a199162_list
-- Function ulam as defined in A002858.
-- Reinhard Zumkeller, Nov 03 2011