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 A243259 #13 Jul 01 2014 20:43:30 %S A243259 99,108,117,198,207,216,297,306,315,396,405,414,495,504,513,594,603, %T A243259 612,693,702,711,792,801,810,972,990,999,1008,1098,1107,1116,1197, %U A243259 1206,1215,1296,1305,1314,1395,1404,1413,1494,1503,1512,1593,1602,1611,1692,1701 %N A243259 Numbers n such that n appears in the sequence x(i) = x(i-1) +/- digitsum(x(i-1)), where even digitsums are added, odd digitsums are subtracted and x(0) = n. %C A243259 The sequence begins with x(0) = n and continues by adding or subtracting the digitsum. 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 A243259 Anthony Sand, <a href="/A243259/b243259.txt">Table of n, a(n) for n = 1..1000</a> %F A243259 x(i) = x(i-1) + digitsum(x(i-1)) * (1 - (digitsum(x(i-1)) mod 2) * 2). %e A243259 digitsum(99) = 18, 18 is even, so 99 + 18 = 117. digitsum(117) = 9, 9 is odd, so 177 - 9 = 108. 108 - 9 = 99, hence 99 belongs to sequence. %e A243259 108 - 9 = 99, 99 + 18 = 117, 117 - 9 = 108, hence 108 is in the sequence. %e A243259 117 - 9 = 108. 108 - 9 = 99. 99 + 18 = 117. %e A243259 198 + 18 = 216. 216 - 9 = 207. 207 - 9 = 198. %Y A243259 Cf. A004207, A243260. %K A243259 nonn,base %O A243259 1,1 %A A243259 _Anthony Sand_, Jun 02 2014