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 A091680 #10 Jul 11 2015 00:52:42 %S A091680 290,78,18,6,3,36,21,19,7,8,4,2,1,-1,-1,-1,-1,-1,-1,-1,-1 %N A091680 Smallest number whose base-4 Reverse and Add! trajectory (presumably) contains exactly n base-4 palindromes, or -1 if there is no such number. %C A091680 Conjecture 1: For each k > 0 the trajectory of k eventually leads to a term in the trajectory of some j which belongs to A075421, i.e., whose trajectory (presumably) never leads to a palindrome. Conjecture 2: There is no k > 0 such that the trajectory of k contains more than twelve palindromes, i.e., a(n) = -1 for n > 12. %C A091680 Base-4 analog of A077594. %H A091680 <a href="/index/Res#RAA">Index entries for sequences related to Reverse and Add!</a> %e A091680 a(4) = 3 since the trajectory of 3 contains the four palindromes 3, 15, 975, 64575 (3, 33, 33033, 3330333 in base 4) and at 20966400 joins the trajectory of 318 = A075421(2) and the trajectories of 1 (A035524) and 2 do not contain exactly four palindromes. %Y A091680 Cf. A075299, A035524, A014192, A075420, A075421, A077594. %K A091680 base,sign %O A091680 0,1 %A A091680 _Klaus Brockhaus_, Jan 28 2004