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 A166080 #12 Aug 09 2025 17:13:21 %S A166080 1,25,85,145,221,265,365,481,545,685,841,925,1105,1405,1513,1625,1985, %T A166080 2245,2665,2813,2965,3281,3445,3785,3961,4141,4325,4705,4901,5305, %U A166080 5513,5725,5941,6161,6385,6613,6845,7081,7565,7813,8065,8321,8845,9113,9385 %N A166080 Nonprimes of the form (k^2+1)/2. %C A166080 Or, 1 together with composite numbers of the form i^2+(i+1)^2. See A012132. - _N. J. A. Sloane_, Feb 29 2020 %C A166080 A166080 U A027862 = A001844. %H A166080 Harvey P. Dale, <a href="/A166080/b166080.txt">Table of n, a(n) for n = 1..1000</a> %F A166080 a(n) = 2n^2 + O(n^2/log n). - _Charles R Greathouse IV_, Mar 21 2014 %e A166080 a(1)=(1^2+1)/2=1; a(2)=(7^2+1)/2=25. %t A166080 Select[(Range[0,150]^2+1)/2,IntegerQ[#]&&!PrimeQ[#]&] (* _Harvey P. Dale_, Aug 09 2025 *) %Y A166080 Cf. A001844, A027862, A012132, A141468. %K A166080 nonn,easy %O A166080 1,2 %A A166080 _Juri-Stepan Gerasimov_, Oct 06 2009 %E A166080 Replaced 6261 by 6161 - _R. J. Mathar_, Oct 07 2009