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 A072139 #4 Jul 11 2015 00:36:41 %S A072139 -1,1,2,3,4,5,6,7,8,9,9,11,9,9,9,9,9,9,9,9,9,9,22,9,9,9,9,9,9,9,9,9,9, %T A072139 33,9,9,9,9,9,9,9,9,9,9,44,9,9,9,9,9,9,9,9,9,9,55,9,9,9,9,9,9,9,9,9,9, %U A072139 66,9,9,9,9,9,9,9,9,9,9,77,9,9,9,9,9,9,9,9,9,9,88,9,9,9,9,9,9,9,9,9,9,99,99,101,99 %N A072139 Last term of the preperiodic part of the 'Reverse and Subtract' trajectory of n, or -1 if the trajectory is completely periodic. %C A072139 'Reverse and Subtract' (cf. A072137) is defined by x -> |x - reverse(x)|. For small n the positive terms are the first palindrome in the trajectory of n, so this sequence is a weak analog of A033865, which uses 'Reverse and Add'. a(1012) = 8712 is the first non-palindrome (cf. A072140). For k in A072140, A072141 or A072142 we have a(k) = -1. %e A072139 a(0) = -1, since 0 -> |0 - 0| = 0, the preperiodic part is empty; a(12) = 9, since 12 -> |12 - 21| = 9. %Y A072139 Cf. A033865, A072137, A072140, A072141, A072142. %K A072139 base,easy,sign %O A072139 0,3 %A A072139 _Klaus Brockhaus_, Jun 24 2002