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 A139131 #12 May 12 2025 02:22:16 %S A139131 1,3,6,10,15,21,14,6,15,55,66,78,91,105,30,34,51,57,190,210,231,253, %T A139131 138,30,65,39,42,406,435,465,62,66,561,595,210,222,703,741,390,410, %U A139131 861,903,946,330,345,1081,282,42,35,255,1326,1378,159,165,770,798,1653,1711 %N A139131 Squarefree kernel of n*(n+1)/2. %H A139131 Reinhard Zumkeller, <a href="/A139131/b139131.txt">Table of n, a(n) for n = 1..1000</a> %F A139131 a(n) = A007947(A000217(n)). %F A139131 a(A014601(n)) = A078636(A014601(n)). %F A139131 a(n) = A078636(n) / A014695(n). %t A139131 Times@@(Transpose[FactorInteger[#]][[1]])&/@Accumulate[Range[60]] (* _Harvey P. Dale_, Jun 06 2013 *) %o A139131 (PARI) a(n) = vecprod(factor(n*(n+1)/2)[, 1]); \\ _Amiram Eldar_, May 12 2025 %Y A139131 Cf. A000217, A007947, A014601, A014695, A078636. %K A139131 nonn,easy %O A139131 1,2 %A A139131 _Reinhard Zumkeller_, Apr 10 2008