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 A089906 #20 Mar 26 2024 14:46:05 %S A089906 0,10,270,4650,66430,864210,10641990,126419770,1464197550,16641975330, %T A089906 186419753110,2064197530890,22641975308670,246419753086450, %U A089906 2664197530864230,28641975308642010,306419753086419790 %N A089906 Sum of digits of numbers between 0 and (4/9)*(10^n-1). %C A089906 From a suggestion of _Yalcin Aktar_ %H A089906 <a href="/index/Rec#order_04">Index entries for linear recurrences with constant coefficients</a>, signature (22,-141,220,-100). %F A089906 a(n) = s(4, n-1) where s(a, k)=a*(k+1)+a^2*sum(i=0, k, i*10^(k-i))+sum(i=0, k, 5*a*(9*(k-i)+a- 1)*10^(k-i-1)). %F A089906 a(n) = (2*(-55*(-1+10^n)+9*(10+9*10^n)*n))/81. G.f.: 10*x*(12*x^2+5*x+1) / ((x-1)^2*(10*x-1)^2). - _Colin Barker_, Jun 14 2013 %Y A089906 Cf. A089903, A089904, A089905, A087330, A089907, A089908, A089909, A034967. %K A089906 nonn,base,easy %O A089906 0,2 %A A089906 _Benoit Cloitre_, Nov 14 2003