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 A345019 #17 Apr 24 2024 19:06:05
%S A345019 1,2,8,9,11,12,18,19,21,22,28,29,31,32,38,39,41,42,48,49,51,52,58,59,
%T A345019 61,62,68,69,71,72,78,79,81,82,88,89,91,92,98,99,101,102,108,109,111,
%U A345019 112,118,119,121,122,128,129,131,132,138,139,141,142,148,149,151,152,158,159
%N A345019 Numbers whose last digit is refactorable.
%C A345019 Numbers ending in 1, 2, 8 or 9.
%H A345019 Wikipedia, <a href="http://en.wikipedia.org/wiki/Refactorable_number">Refactorable number</a>.
%H A345019 <a href="/index/Rec#order_05">Index entries for linear recurrences with constant coefficients</a>, signature (1,0,0,1,-1).
%F A345019 a(n) = 10*floor((n-1)/4)+(10+(-1)^n+7*cos(n*Pi/2)-7*sin(n*Pi/2))/2.
%F A345019 Sum_{n>=1} (-1)^(n+1)/a(n) = sqrt((1+1/sqrt(5))/2)*Pi/5. - _Amiram Eldar_, Apr 15 2023
%F A345019 G.f.: x*( 1+x+6*x^2+x^3+x^4 ) / ( (1+x)*(1+x^2)*(x-1)^2 ). - _R. J. Mathar_, Mar 25 2024
%F A345019 a(n) = a(n-1) + a(n-4) - a(n-5). - _Wesley Ivan Hurt_, Apr 24 2024
%e A345019 158 is in the sequence since 8 is a refactorable number.
%t A345019 Table[10*Floor[(n - 1)/4] + (10 + (-1)^n + 7*Cos[n*Pi/2] - 7*Sin[n*Pi/2]) / 2, {n, 100}]
%Y A345019 Cf. A033950 (refactorable numbers).
%K A345019 nonn,easy,base
%O A345019 1,2
%A A345019 _Wesley Ivan Hurt_, Jun 05 2021