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 A258080 #17 Jul 21 2025 19:08:27
%S A258080 1,1,5,19,1,17,53,43,29,31,79,29,179,199,11,69,169,317,139,361,185,
%T A258080 497,331,149,601,439,263,73,739,581,411,229,35,1019,871,713,545,367,
%U A258080 179,1621,1495,1361,1219,1069,911,745,571,389,199,1,2447,2337,2221,2099
%N A258080 a(n) = ((n concatenated with n+1) + (n+1 concatenated with n)) mod n*(n+1).
%H A258080 Peter J. C. Moses, <a href="/A258080/b258080.txt">Table of n, a(n) for n = 1..2000</a>
%t A258080 Map[Mod[FromDigits[Flatten[{#1,#2}]]+FromDigits[Flatten[{#2,#1}]]&[IntegerDigits[#+1],IntegerDigits[#]],# (#+1)]&,Range[100]] (*_Peter J. C. Moses_, May 19 2015*)
%t A258080 Mod[(#[[1]]*10^IntegerLength[#[[2]]]+#[[2]])+(#[[2]]*10^IntegerLength[#[[1]]]+#[[1]]),Times@@#]&/@Partition[Range[60],2,1] (* _Harvey P. Dale_, Jul 21 2025 *)
%o A258080 (PARI) a(n) = (eval(concat(Str(n), Str(n+1))) + eval(concat(Str(n+1), Str(n)))) % (n*(n+1)); \\ _Michel Marcus_, Jun 04 2015
%Y A258080 Cf. A001704 (concatenation {n,n+1}).
%K A258080 nonn,base
%O A258080 1,3
%A A258080 _Vladimir Shevelev_, May 19 2015
%E A258080 More terms from _Peter J. C. Moses_, May 19 2015