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 A243260 #16 Jul 02 2014 17:15:48 %S A243260 81,90,99,171,180,189,261,270,279,351,360,369,441,450,459,531,540,549, %T A243260 621,630,639,711,720,729,801,810,819,1071,1080,1089,1161,1170,1179, %U A243260 1251,1260,1269,1341,1350,1359,1431,1440,1449,1521,1530,1539,1611,1620,1629 %N A243260 Numbers n such that n appears in the sequence x(i) = x(i-1) +/- digitsum(x(i-1)), where even digit sums are subtracted, odd digit sums are added and x(0) = n. %C A243260 The auxiliary sequence begins with x(0) = n and continues by adding or subtracting the digit sum. When the digitsum(x(i-1)) is even, x(i) = x(i-1) - digitsum(x(i-1)), otherwise x(i) = x(i-1) + digitsum(x(i-1)). %H A243260 Anthony Sand, <a href="/A243260/b243260.txt">Table of n, a(n) for n = 1..1000</a> %F A243260 x(i) = x(i-1) + digitsum(x(i-1)) * -(1 - (digitsum(x(i-1)) mod 2) * 2). %e A243260 digitsum(81) = 9, 9 is odd, so 81 + 9 = 90. 90 + 9 = 99. digitsum(99) = 18, 18 is even, so 99 - 18 = 81, so 81 is in the list. %e A243260 90 + 9 = 99. 99 - 18 = 81. 81 + 9 = 90. %e A243260 99 - 18 = 81. 81 + 9 = 90. 90 + 9 = 99. %e A243260 171 + 9 = 180. 180 + 9 = 189. 189 - 18 = 171. %e A243260 180 + 9 = 189. 189 - 18 = 171. 171 + 9 = 180. %e A243260 Starting with n=81, we have 81+9(odd)=90, 90+9(odd)=99, 99-18(even)=81 for the auxiliary x(i) sequence; so 81 is in the main sequence; starting with n=90 or 99 will lead to the same cycle loop, so 90, 99 are also in this sequence. %Y A243260 Cf. A243259, A004207. %K A243260 nonn,base %O A243260 1,1 %A A243260 _Anthony Sand_, Jun 02 2014