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 A079215 #3 Mar 30 2012 18:51:39 %S A079215 1,121,12121,12321,1212121,1212321,1232121,1232321,1234321,121212121, %T A079215 121212321,121232121,121232321,121234321,123212121,123212321, %U A079215 123232121,123232321,123234321,123432121,123432321,123434321,123454321 %N A079215 Catalan paths: numbers starting with 1 and ending with 1 where each digit is positive and adjacent digits differ by 1. %C A079215 a(6917) cannot easily be written in decimal. %F A079215 a(n) =A079214(n)+(10^(2+floor[log_10(A079214(n))])-1)/9. %e A079215 Can be constructed from A063171 by starting from a leading 1 and adding 1 to previous digit of a(n) if next digit of A063171 is 1 and subtracting 1 if next digit is 0: e.g. A063171(3)=1100, so digits of a(3) are 1, 1+1=2, 2+1=3, 3-1=2 and 2-1=1 giving a(3)=12321. %Y A079215 Cf. A063171, A079214. a(n) has A000108(m) values with 2m+1 digits. Among the values with 2m+1 digits, the digit k appears a total of A039598(m, k-1) times. %K A079215 base,nonn %O A079215 0,2 %A A079215 _Henry Bottomley_, Jan 02 2003