A055456 a(n) is the smallest number which is not the sum of exactly 1 or of n earlier terms.

1, 3, 2, 13, 4, 5, 6, 7, 8, 9, 10, 11, 12, 183, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71

Offset: 1

Author

Henry Bottomley, May 19 2000

Keywords

• easy
• nonn

Examples

			a(3)=2 because 1 is already in the sequence, 2 has not yet appeared (i.e., is not the sum of 1 earlier term), and the sum of 3 earlier terms is 3, 5 or 7.
a(4)=13 because 1, 2, and 3 have already appeared and the sum of 4 earlier terms could be any integer from 4 through 12.
		

Crossrefs

Cf. A035334. a(n) is not n-1 iff n-1 is in A024556 or equivalently in A002065.

Formula

If n-1 > 0 has not already appeared in sequence then a(n) = n-1, otherwise a(n) = n^2 - n + 1.