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 A074993 #14 Dec 12 2019 14:51:49 %S A074993 0,6,11,17,22,28,33,39,44,455,505,556,606,657,707,758,808,859,909,960, %T A074993 1010,1061,1111,1162,1212,1263,1313,1364,1414,1465,1515,1566,1616, %U A074993 1667,1717,1768,1818,1869,1919,1970,2020,2071,2121,2172,2222,2273,2323,2374 %N A074993 a(n) = floor((concatenation of n, n+1)/2). %C A074993 The first differences follow a pattern. Odd-indexed terms and even-indexed terms form separate A.P.s with the same common difference for all n except n = 10^k -1. The corresponding common differences are the repunits = (10^(d+1)-1)/9 where d = the number of digits in n. %t A074993 cc[n_]:=Floor[FromDigits[Join[IntegerDigits[n],IntegerDigits[n+1]]]/2]; Array[cc,40,0] (* _Harvey P. Dale_, Nov 11 2011 *) %Y A074993 Cf. A001704, A074991, A074992, A074994, A074995, A074996, A074997, A073086, A074999, A127421. %K A074993 base,easy,nonn %O A074993 0,2 %A A074993 _Amarnath Murthy_, Aug 31 2002