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 A090618 #5 Jun 07 2016 16:48:12 %S A090618 0,0,0,0,0,0,1,1,1,2,2,2,2,2,2,3,3,3,4,4,4,4,4,4,5,5,5,6,6,6,7,7,7,7, %T A090618 7,7,8,8,8,9,9,9,9,9,9,10,10,10,11,11,11,11,11,11,13,13,13,13,13,13, %U A090618 14,14,14,15,15,15,15,15,15,16,16,16,17,17,17,17,17,17,18,18,18,20,20,20,20 %N A090618 Highest power of 9 dividing n!. %F A090618 a(n) =A090622(n, 9) =[A054861(n)/2] =[([n/3]+[n/9]+[n/27]+[n/81]+...)/2]. %e A090618 a(9)=2 since 9!=362880=9^2*4480. %t A090618 IntegerExponent[Range[0,90]!,9] (* _Harvey P. Dale_, Jun 07 2016 *) %Y A090618 Cf. A011371, A054861, A090616, A027868, A054896, A090617. %K A090618 nonn %O A090618 0,10 %A A090618 _Henry Bottomley_, Dec 06 2003