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 A146209 #22 Dec 28 2023 19:34:53 %S A146209 10,15,26,30,34,35,39,42,51,55,58,65,66,70,74,78,79,82,85,87,91,95, %T A146209 102,105,106,110,111,114,115,119,122,123,130,138,142,143,145,146,154, %U A146209 155,159,165,170,174,178,182,183,185,186,187,190,194,195 %N A146209 Integers a(n) for which the factorization in the real quadratic field Q(sqrt(a(n))) is not unique. %C A146209 The class number of Q(sqrt(a(n))) is greater than 1. %C A146209 Contains A029702, A053330 and A051990 as subsequences. See A219361 for positive integers D for which Q(sqrt D) is a UFD. - _M. F. Hasler_, Oct 30 2014 %D A146209 Z. I. Borevich and I. R. Shafarevich, Zahlentheorie. Birkhäuser Verlag, Basel und Stuttgart (1966). %H A146209 Vincenzo Librandi, <a href="/A146209/b146209.txt">Table of n, a(n) for n = 1..1000</a> %e A146209 For n = 6, a(6) = 35 since 35 is the sixth positive squarefree integer u for which the factorization in Q(sqrt(u)) is not unique. %t A146209 Select[Range[200], SquareFreeQ[#] && NumberFieldClassNumber[Sqrt[#]] > 1 &] (* _Alonso del Arte_, Sep 05 2012 *) %Y A146209 Cf. A003172. %K A146209 nonn %O A146209 1,1 %A A146209 _Pahikkala Jussi_, Oct 28 2008