This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A289642 #8 Jul 10 2017 04:27:12 %S A289642 1,2,3,4,5,6,7,8,9,9,8,7,6,5,4,3,2,1 %N A289642 Number of 2-digit numbers whose digits add up to n. %C A289642 The 2-digit numbers distributed according to the sum of their digits n. %C A289642 Symmetrical sequence; a(n) = a(19 - n). %F A289642 G.f.: (1 - x^10)*(x - x^10)/(1 - x)^2. %F A289642 a(n) = (19-abs(n-9)-abs(n-10))/2 for n=1..18. - _Wesley Ivan Hurt_, Jul 09 2017 %e A289642 n(5) = 5 because there are 5 numbers whose digits sum = 5 (14, 23, 32, 41, 50). %Y A289642 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. %K A289642 fini,full,nonn,base %O A289642 1,2 %A A289642 _Miquel Cerda_, Jul 09 2017