A289642 Number of 2-digit numbers whose digits add up to n.

1, 2, 3, 4, 5, 6, 7, 8, 9, 9, 8, 7, 6, 5, 4, 3, 2, 1

Offset: 1

Author

Miquel Cerda, Jul 09 2017

Keywords

• fini
• full
• nonn
• base

Comments

The 2-digit numbers distributed according to the sum of their digits n.

Symmetrical sequence; a(n) = a(19 - n).

Examples

			n(5) = 5 because there are 5 numbers whose digits sum = 5 (14, 23, 32, 41, 50).
		

Crossrefs

Cf. A071817 (3-digit numbers), A090579 (4-digit numbers), A090580 (5-digit numbers), A090581 (6-digit numbers), A278969 (7-digit numbers), A278971 (8-digit numbers), A289354 (9-digit numbers), A053188, A074989, A004739, A066635, A154840, A249121.

Formula

G.f.: (1 - x^10)*(x - x^10)/(1 - x)^2.

a(n) = (19-abs(n-9)-abs(n-10))/2 for n=1..18. - Wesley Ivan Hurt, Jul 09 2017