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 A352990 #11 Apr 20 2022 00:08:05 %S A352990 2,6,12,21,33,45,52,63,136,162,201,205,208,225,245,253,301,304,344, %T A352990 441,494,531,550,637,697,720,742,806,901,910,918,1078,1233,1242,1274, %U A352990 1333,1376,1540,1566,1573,1625,1680,1695,1792,1863,1909,2025,2041,2107,2295,2466,2497,2774,2896,2926,2965 %N A352990 Numbers k such that the k-th triangular number == 1 mod the integer log of k. %C A352990 Numbers k such that A000217(k) == 1 (mod A001414(k)). %H A352990 Robert Israel, <a href="/A352990/b352990.txt">Table of n, a(n) for n = 1..10000</a> %e A352990 a(3) = 12 = 2*2*3 is a term because 12*13/2 = 78 == 1 (mod 2+2+3 = 7). %p A352990 filter:= proc(n) local t; (n*(n+1)/2) mod add(t[1]*t[2],t=ifactors(n)[2]) = 1 end proc: %p A352990 select(filter, [$2..3000]); %t A352990 Select[Range[3000], Mod[#*(# + 1)/2, Plus @@ Times @@@ FactorInteger[#]] == 1 &] (* _Amiram Eldar_, Apr 14 2022 *) %Y A352990 Cf. A000217, A001414, A352989. %K A352990 nonn %O A352990 1,1 %A A352990 _J. M. Bergot_ and _Robert Israel_, Apr 13 2022