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 A337208 #29 Mar 26 2025 09:01:59
%S A337208 1,4,6,17,28,39,50,61,72,83,94,109,120,131,142,153,164,175,186,197,
%T A337208 199,210,221,232,243,254,265,276,287,298,300,311,322,333,344,355,366,
%U A337208 377,388,399,401,412,423,434,445,456,467,478,489,500,502,513,524,535,546,557
%N A337208 Indices m of repunits R_m that are Colombian (or self) numbers.
%H A337208 David A. Corneth, <a href="/A337208/b337208.txt">Table of n, a(n) for n = 1..10000</a>
%H A337208 Shyam Sunder Gupta, <a href="https://doi.org/10.1007/978-981-97-2465-9_9">On Some Marvellous Numbers of Kaprekar</a>, Exploring the Beauty of Fascinating Numbers, Springer (2025) Ch. 9, 275-315.
%o A337208 (PARI) upto(n)= {my(res = List()); for(i = 1, n, if(is(i), listput(res, i); print1(i", "))); res}
%o A337208 is(n) = {c = 10^n \ 9; is_A003052(c)}
%o A337208 is_A003052(n)={for(i=1, min(n\2, 9*#digits(n)), sumdigits(n-i)==i && return); n} \\ from _M. F. Hasler_, Mar 20 2011
%Y A337208 Cf. A002275 (repunits), A003052 (Colombian (or self)), A004022 (repunit primes), A004023 (indices of repunit primes), complement of A337139.
%K A337208 nonn,base
%O A337208 1,2
%A A337208 _David A. Corneth_, Aug 19 2020