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 A081770 #16 Nov 03 2020 03:41:27 %S A081770 4,12,20,28,44,52,60,68,76,84,92,116,124,132,140,148,156,164,172,188, %T A081770 204,212,220,228,236,244,260,268,276,284,292,308,316,332,340,348,356, %U A081770 364,372,380,388,404,412,420,428,436,444,452,460,476,492,508,516,524 %N A081770 Numbers twice their squarefree kernel (A007947). %C A081770 From _Amiram Eldar_, Nov 02 2020: (Start) %C A081770 Numbers k such that A280292(k) = 2. %C A081770 The asymptotic density of this sequence is 1/Pi^2 (A092742). (End) %H A081770 Reinhard Zumkeller, <a href="/A081770/b081770.txt">Table of n, a(n) for n = 1..10000</a> %F A081770 a(n) = 2*A039956(n) = 4*A056911(n). %e A081770 84=2*2*3*7=2*(2*3*7)=2*rad(84), therefore 84 is a term. %t A081770 4 * Select[Range[1, 100, 2], SquareFreeQ] (* _Amiram Eldar_, Nov 02 2020 *) %o A081770 (PARI) is(n)=n%8==4 && issquarefree(n/4) \\ _Charles R Greathouse IV_, Jul 09 2013 %o A081770 (Haskell) %o A081770 a081770 n = a081770_list !! (n-1) %o A081770 a081770_list = filter ((== 1) . a008966 . (`div` 4)) a017113_list %o A081770 -- _Reinhard Zumkeller_, Jul 13 2013 %Y A081770 Cf. A001414, A005117, A007947, A008472, A008966, A013929, A039956, A056911, A092742, A280292. %Y A081770 Subsequence of A017113. %K A081770 nonn,easy %O A081770 1,1 %A A081770 _Reinhard Zumkeller_, Apr 10 2003